11 ms·

In defence of swap: common misconceptions

150 points by c4urself 9y ago

bhouston 9y ago
On windows I have found it necessary to disable swap to keep myself efficient. Many times Ive had applications decide to allocate massive amounts of memory and then it leads to my system slowing down with tons of swap activity. In nearly all cases, I didn't want my system to try its best to handle these massive memory requests but rather it should have just killed the offending application. Often in these failure scenarios the swap goes nuts and my computer becomes unresponsive that it takes a long time to even get to kill the bad actor. Thus I disabled swap and I never had these unresponsive issues. I run with 32GB of ram so generally well behaved applications never run into memory issues. Some applications that would cause issues would be too many VirtualBox instances that use more than available memory. A text editor that chocks trying to open a >1GB text file (looking at you, the new JS-based editors.)
- maxxxxx 9y ago
  Windows is terrible at swapping. As soon as you hit max RAM performance suffers a lot. Even if you never use inactive apps.
  - mnx 9y ago
    Yep. When I was using win7 with 2gb of ram, I was basically managing it manually - as soon as memory usage hit 90-95%,I was shutting apps down. If I let it swap, it would often hang completely.
kstenerud 9y ago
So, in other words, if you have enough memory for your workload that you won't run out, there's no benefit to having swap space (i.e. you've wasted money on memory you don't need). But if you DO have swap space, there won't be a performance hit (at least not under Linux) because it will only swap out some rarely used pages and then sit there doing nothing. So, in the general case, it's better to have it and not need it than need it and not have it.
- Anderkent 9y ago
  > So, in other words, if you have enough memory for your workload that you won't run out, there's no benefit to having swap space (i.e. you've wasted money on memory you don't need). No, that's the opposite. If you have enough memory for your workload that you won't run out, swapping lets you use more memory for disk cache (instead of keeping unaccessed anonymous pages in real ram). Unless by "won't run out" you mean "never have to throw away a disk cache page", which seems very unrealistic.
  - clarry 9y ago
    > If you have enough memory for your workload that you won't run out, swapping lets you use more memory for disk cache Except that it doesn't happen in practice, on my systems anyway. If you have plenty of memory, you can keep all your programs in it and as much as the system wants to cache and still not run out. The theory says that swap effectively buys you some memory to spend on more important things (than what the system chooses to page out). So does buying more memory. > Unless by "won't run out" you mean "never have to throw away a disk cache page", which seems very unrealistic. I have an instance of top running on my desktops & laptops all the time. I never see cache using up all of the memory. total used free shared buff/cache available Mem: 7848 1523 5827 53 497 6033
    - Anderkent 9y ago
      Are you talking desktop workflows here? If this is a service, why are you paying for RAM that never gets used? Get a smaller instance.
      - clarry 9y ago
        > Are you talking desktop workflows here? Yes [I explicitly spelled out desktops & laptops] but it's been true on my servers too. > If this is a service, why are you paying for RAM that never gets used? Get a smaller instance. Not all providers are so flexible. I may want a server with more CPU & traffic and some additional disk space. Going there gets me more RAM too. It turns out these services are constrained by the real hardware. If I'm getting a box with enough CPU for my needs, well they are not going to a shop to buy that special box for me, they use what they have and what they have comes with plenty of RAM too.
loeg 9y ago
All of this boils down to "because buying more disk is cheaper than buying more RAM" and "avoid memory contention." The author discusses the situation as if the quantity of RAM is fixed and swap can be added (or not). But that isn't the only possibility — you can also add more RAM (it's just expensive). For the same number of GB of RAM+swap vs just RAM, there is no reason to prefer the option with swap.
- Anderkent 9y ago
  Sure, but why wouldn't you run some swap with your much bigger ram anyway? In the end the core idea is: sometimes you have anonymous memory that is accessed so rarely that you'd rather have an extra disk cache page. If you assume that the kernel is not paging out memory that you actually use when not under pressure, swap doesn't hurt you.
  - clarry 9y ago
    > Sure, but why wouldn't you run some swap with your much bigger ram anyway? If you don't need it, you don't need it. The other question is: how much swap exactly should I have? And why wouldn't I just add that much RAM instead? > In the end the core idea is: sometimes you have anonymous memory that is accessed so rarely that you'd rather have an extra disk cache page. That's the theory. In practice I always have more than enough RAM for all the cached pages the system wants to cache. On my laptop right now (booted today), I have 500MB of cached pages and 5.8 gigabytes of free memory. On my server (booted 499 days ago) I have 700MB of cached pages and 6 gigabytes of free memory. If I were running out of memory [be it for cache or applications], I'd prefer to have more RAM than add swap. Yes, I keep calling it emergency memory. > If you assume that the kernel is not paging out memory that you actually use when not under pressure, swap doesn't hurt you. 1) Bad assumption 2) it doesn't help you either, so why bother? Actually I might have a use for that disk space. In that case the swap just hurts.
    - Anderkent 9y ago
      >If you don't need it, you don't need it. The other question is: how much swap exactly should I have? And why wouldn't I just add that much RAM instead? Because adding RAM costs money, adding swap space is a config setting. It looks to me that your boxes are idle. Sure, if you're not doing any work, it doesn't matter... >1) Bad assumption 2) it doesn't help you either, so why bother? Actually I might have a use for that disk space. In that case the swap just hurts. 1) Is it? Can you source that claim somehow? 2) It does help you, that's the point. It makes your disk reads faster 3) Your answer to needing memory is 'buy more ram', but you 'may have a use for that disk space'? Buy more disk.
nicklaf 9y ago
I have an older Chromebook (c720), which is really quite memory starved (2GB RAM), and have experienced ChromeOS completely frying the SSD simply through prolonged tab-heavy swapping. Now, I've replaced the SSD and installed a non-Google Linux distro, and would like to limit the amount of swapping Firefox can do. I had been planning to simply use cgroups' memory features to limit the amount of memory consumed by Firefox processes, but if I am to understand the article (which I admit I didn't read in full detail), I should also be able to tune swapping to limit the actual amount of swapping that takes place, avoiding a drastic uptick in SSD wear whenever open too many tabs. That, and perhaps a Firefox extension that suspends background tabs in memory (which I've used before with a certain amount of effectiveness in the pre-WebExtension days).
- bruce_one 9y ago
  Setting the `swappiness` value might be of use to you. Setting a lower value (min is 0, default is 60 iirc, max is 100, so above half) reduces the likelihood of the kernel swapping. The lower the number the fuller ram needs to be before the kernel will start swapping. Hence a low number will mean that less swapping will happen, hence meaning less SSD wear.
  - nicklaf 9y ago
    Thanks, I should simply start with that. Along with a hard limit set by cgroups so that browser tabs start being killed in order to stop the swap from being overwhelmed when memory is being used well beyond its capacity. (I'd be interested to know if 'swappiness' already effectively implements the following system-wide, but here goes.) Now that I think of it, it's not so much the quantity of disk storage being consumed by swap, so much as the number of write / delete transactions. One thus wonders if an approach in which the browser somehow favors a small number of tabs to keep in ram, and then dumps the state of the remaining tabs to disk, might be just as effective, but without worrying about the growth of swap. Then, when the user opens a tab that had been saved to disk, if we crudely assume that the memory consumed by open tabs are roughly comparable in memory use, then we can take care of the whole affair of 'swapping' in a single exchange between memory and disk, where we dump the state of the in-memory tab in order to make way for restoring the saved one. (Or did I just reinvent what swappiness does already? From your description of swappiness, I'm inclined to guess the answer is no. This approach strikes me more akin to using the swapfile as a filesystem, and keeping just a small number of tabs paged into RAM.)
whopa 9y ago
This is an informative and well written article, but seems incomplete in this day and age. In public cloud environments, network attached storage is far more prevalent, so the swap story may be different there (I honestly don't know though). Since the author works at Facebook, he probably lacks experience in this regard.
- Anderkent 9y ago
  Every cloud provider I've worked with (okay, so AWS :P) gives you ephemeral local storage. Obvioulsy you don't swap onto a network drive.
  - akerl_ 9y ago
    Even on AWS they're phasing out local storage on new instance types: https://ec2instances.info/ https://ec2instances.info/ (search for ec2 only, but it's the majority of new instance families)
  - whopa 9y ago
    Modern AWS instance types are EBS-only.
    - eikenberry 9y ago
      With the exception of the High IO types (I2/I3). They still get it and the newer instances get NVMe SSDs. In other words they are making it a feature of certain types that would benefit from it.
      - _msw_ 9y ago
        For example, F1 instances have NVMe local instance storage. https://aws.amazon.com/ec2/instance-types/f1/ https://aws.amazon.com/ec2/instance-types/f1/
    - Anderkent 9y ago
      Huh. You're right; it seems for the newer instance types only c1.medium and m1.small get swap mounts. That seems like a mistake by aws; but I guess you can a M3 instead of a M5.
avar 9y ago
Even for those who understand this well, it's historically been really hard to coerce the Linux kernel into applying the right swap policies to your application. As the author notes much of this has been improve by cgroups, and there's always been big hammers like mlock(), even with those things it can be hard to prevent memory thrashing in extreme cases. I've seen swap disabled completely by people who understood how it worked as a last result because of that. It's always seemed to me that this was mainly a problem of the kernel configuration being too opaque. Why can't you configure on a system-wide basis that you can use swap e.g. only for anonymous pages and for nothing else? Similarly it would be nice to have a facility for the OOMkiller to call out to userspace (it would need dedicated reserved memory for this) to ask a configured user program "what should I kill?". You might also want to do that when you have 10G left, not 0 bytes.
- Hello71 9y ago
  Android has this last bit. as explained at https://www.youtube.com/watch?v=ikZ8_mRotT4&t=2145 https://www.youtube.com/watch?v=ikZ8_mRotT4&t=2145 (linked in the article) though, Linux does not have the facilities presently to determine when you have "10G left" in a way that applies across all system configurations.
- caf 9y ago
  Swap is only used for anonymous pages (well, and dirty private file pages, which are basically the same thing).
wahB4vai 9y ago
I'm a big fan of determinism and service uniformity. Having that rarely used and response time critical function/data/whatever swapped out increases service time variation at best and complicates all worst case response time calculations at least. I understand from the land of JIT compilers, garbage collectors, and oversubscribed everything that this is not much of a substantial concern as these features are already traded away. The swap may be the best case in a bad situation. I would argue along the lines of don't be in a bad situation... I'm looking at you 8 of 16 GB used on cold boot Mac laptops... Looking at you with indignation and rancor Chrome.
- caf 9y ago
  One of the article's points is that running without swap doesn't necessarily alleviate that. The rarely-used code pages of your rarely-used but response time critical daemon can just as easily be dropped from the page cache and have to be refaulted in from disk, and in fact that's more likely if there isn't swap available to stow the dirty anonymous pages from the cron daemon that wakes up once a day or whatever. The solution for your rarely-used but response time critical daemon is for it to mlock() its critical data and code pages into memory, which works regardless of whether or not you have swap available. (Or, alternatively, use one of the cgroup controllers that the article alludes to, to give the critical daemon and related processes memory unaffected by memory pressure elsewhere in the system).
  - AstralStorm 9y ago
    No they cannot. Code that is loaded into memory stays in memory when three is no swap. (Mlock is to prevent swap out or compression - major faults.) The exception would be lazy library loading. BIND_NOW is your friend in this case. Essentially having no swap is similar to having everything mlocked - no major faults can happen except with mmapped files which will just use direct disk IO. If you mean disk caches, when have you seen a multigigabyte executable?
    - caf 9y ago
      Code that is loaded into memory stays in memory when three is no swap. That is not true. In the normal case (absent debugging, JIT, self-modifying code etc), pages of executable code are clean, shared mappings so they do not interact with swap at all. As clean, shared mappings they are eligible to be dropped from the page cache in the same way as other clean file mapped pages. (Your executable code pages essentially are mmapped files. )
deleted 9y ago
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slaymaker1907 9y ago
I recently reenabled swap on my Windows machine due to frequent OOM, even with 16GB of RAM while playing Overwatch and browsing on Firefox. It seems like both of these programs allocate vast swaths of memory but then do not actually use that memory very heavily. After I turned swap back on, I did not notice any degradation in performance but my system stability skyrocketed.
- wilun 9y ago
  Windows has vastly different policies for RAM allocation and commit than Linux. Windows basically does not overcommit while Linux systems not only does it by default but quite depend on it for various loads to work properly. In consequence, the userspace has a tendency to handle RAM differently, but there is no magic: if programs are trying to allocate twice the amount of RAM and then only use half of it, Windows with a swap that can be large enough will work perfectly while without swap the allocation will fail. Under Linux, the situation is less clear: without swap it will succeed (well it depends on the fine details of overcommit that are selected, but you get the idea) although if you really then use all that RAM, the OOM killer will start to more or less "randomly" kill "any" process to cope with the lack of RAM (as a last resort measure though; the caches and buffers are flushed before, etc.)
  - Asooka 9y ago
    Windows also has two syscalls - one to reserve memory and one to commit memory. You can reserve+commit at once, but you can also just reserve a chunk of virtual memory that you commit at a later time. Accessing pages that aren't committed is a segfault. So you can say "I might need up to X contiguous bytes of virtual memory" and then commit as you go. IIRC, Windows will let you over-reserve, but not over-commit. Edit: sorry, not two syscals, it's an option to the malloc-equivalent - VirtualAlloc
CoconutPilot 9y ago
Swap was a great idea, but its time is gone. Swap doesn't make sense anymore, hard drives have not scaled and kept up with the improvements in RAM. In the Pentium 1 era EDO RAM maxed out at 256MB/s and hard disk xfer was 10MB/s. Common RAM size was 16MB. In today's era DDR4 maxes out at 32GB/s and hard disk xfer is 500 MB/s. Common RAM size is 16GB. RAM xfer rate has grown is 320x. RAM capacity has grown 100x. Disk xfer rate has grown 50x. Swap is no longer a useful tool.
- Dylan16807 9y ago
  The purpose of swap is not running things out of it. The purpose is shoveling unused data out of memory. And for that it doesn't need to be particularly fast. Swap has a lot less purpose in a world without memory leaks and extraneous functions. But in practice it's quite good at getting several gigabytes of unnecessary data out of the way, so ram can be used properly. Swap, well-used, should only take up a few percent of the drive's bandwidth.
  - lesserknowndan 9y ago
    This reply was a much better explanation than the entire original article, or to summarize “Because memory leaks”.
  - tmyklebu 9y ago
    > The purpose of swap is not running things out of it. The purpose is shoveling unused data out of memory. And for that it doesn't need to be particularly fast. You can't detect a priori whether data is "unused." If you guess wrong a few times in a row, you get the familiar pattern where your Linux box is unresponsive to everything and needs to be bounced. If you could detect whether data is rarely used, swap still isn't necessary. Applications can mmap() a file and use that region for "rarely used data" if such is known in advance. Extraneous functions should be backed by the executable in the common case. In the JIT case, they probably won't be JITted anyway. I still think the OOM killer is less intrusive than swapping to disk. It kills some, but not all, of the processes on the machine get killed. The system pretty reliably comes back to life in less time than it takes a human to diagnose the problem and bounce the system. As a bonus, no human needs to get involved.
dboreham 9y ago
We've disabled swap (by not configuring a swap partition) on every server we've deployed since 2009. It's a little irritating to have to manually remove the swap partition from various Linux' "server" default install options even today. Of course this means I'm still installing on bare metal that I own, so...dinosaur.
perlgeek 9y ago
> Under temporary spikes in memory usage > With swap: We’re more resilient to temporary spikes, but in cases of severe memory starvation, the period from memory thrashing beginning to the OOM killer may be prolonged. We have more visibility into the instigators of memory pressure and can act on them more reasonably, and can perform a controlled intervention. Somehow that doesn't resonate with my experience. I tend to remember the cases where I can't even SSH into the box, because the fork in sshd takes minutes, as does spawning the login shell. I'd really like some way to have swap, but still loosen the OOM killer on the biggest memory hog when the system slows down to a crawl. I haven't found that magic configuration yet.
- XorNot 9y ago
  Swap iotime quotas maybe? I suspect the solution is a lot more complicated though - what I really want is a way to wall off my UI so it stays responsive during swap thrashing and let's me react to the situation.
- rfjakob 9y ago
  There's a userspace daemon that does that: https://github.com/rfjakob/earlyoom https://github.com/rfjakob/earlyoom (I am the author)
- codesnik 9y ago
  I remember not being able to SSH on the box too, but this stopped be the case (for me at least) some five years ago - I was able to login to heavy swap trashing boxes without any problem. I thought that something was changed in contemporary linux distros. Either login shell was given much more priority or something like that.
- JdeBP 9y ago
  Well the article does suggest a mechanism to apply. As for the problem with SSH and login: You might well find that it is not the fork that is the problem. You might well be surprised at how much chaff is run by a login shell, or even by non-login shells. A case in point: I recently reduced the load on a server system that involved lots of SCP activity by noticing that, thanks to RedHat bug #810161, every SCP session even though it was a non-login non-interactive shell was first running a program to enumerate the PCI bus, to fix a problem with a Cirrus graphics adapter card that the machine did not have on a desktop environment that the machine did not have. This driven by /etc/bashrc sourcing /etc/profile.d/* . * https://github.com/FedoraKDE/kde-settings/blob/F-26/etc/profile.d/qt-graphicssystem.sh https://github.com/FedoraKDE/kde-settings/blob/F-26/etc/prof...
cosarara97 9y ago
In my experience, a misbehaving linux system that's out of RAM and has swap to spare will be unusably slow. The process of switching to a tty, logging in, and killing whatever the offending process is can easily take a good 15 minutes. Xorg will just freeze. Oh, and hopefully you know what process it is, else good luck running `top`. Until this is fixed, I'll just keep running my systems with very small amounts of swap (say, 512MB in a system with 16GB of RAM). I'd rather the OOM killer kick in than have to REISUB or hold down the power button. Some benchmarks with regards to the performance claims would be nice.
- kzrdude 9y ago
  What function does the 0.5GB swap have?
  - petecox 9y ago
    I just wish Linux distro installers would make opting out the default option; no, I don't want to swap on my SSD. The last time I installed a distro, I still had to select the manual option for partitioning. With an 8 Gig stick in my NUC, for normal desktop usage it never goes above 3.
    - Karunamon 9y ago
      And even then; they provision an absurd amount of it. I just did a fresh Ubuntu install. On a machine with 32 gigs ram, it creates a 32 gig swap partition by default!
      - KozmoNau7 9y ago
        They probably reason that you want your system to be able to hibernate, plus storage is cheap. I have 8GB of swap for the 8GB in my laptop, for that reason. On my desktop with 16GB, the w 2GB of swap it has is sometimes too little, and everything grinds to a halt.
        9y ago
ohazi 9y ago
Does hibernating via a swap file work reasonably well yet? I haven't had a chance to try this out yet, but that's the main reason I still have a swap partition on my laptop.
- gerdesj 9y ago
  Well done mate - you are the first person to mention this here. It was also only briefly mentioned in the article. Yes, hibernation does work well and it requires swap. Personally, I set a swap partition equal to RAM + 512MB on systems that I want to hibernate on. Linux also supports swap files and this might be handy: https://wiki.debian.org/Hibernation/Hibernate_Without_Swap_Partition https://wiki.debian.org/Hibernation/Hibernate_Without_Swap_P...
Aardwolf 9y ago
In defence against swap on my personal computer: -PCs have a lot of RAM now -When you allocate that much memory it's usually a bug in your own code like a size_t that overflowed. I never saw programs I would actually want to use try to allocate that much -When using swap instead of ram, everything becomes so slow that you're screwed anyway. The UI doesn't even respond fast enough to kill whatever tries to use all that memory. -How common is a situation where you need more memory than your ram size yet less than ram+swap size in a useful way? Usually if something needs a lot, it's really lot (and as mentioned above not desirable) -Added complexity of making extra partition -Added complexity if you want to use full disk encryption -I do the opposite of using disk as ram: I put /tmp in a ramdisk of a few gigs -Disks are slow and fast ssd's are expensive so you would't want to sacrifice their space (maybe if this changes some day...)
- mort96 9y ago
  Regarding the complexity of making an extra partition: I completely agree, having a separate partition for swap is annoying. Luckily, Linux supports swap files; all my systems (where I had to manually set up swap), I just create a file called /swapfile. I imagine this would solve the full disk encryption complexity too.
- IncRnd 9y ago
  Exactly! Also, the person who goes from 32G of RAM to 256G of RAM is going to run without swap.
  - deleted 9y ago
    [deleted]
  - Symbiote 9y ago
    I have 4GiB swapfiles on the cluster nodes I manage, which have 512GiB RAM. It's hardly used, around 300MiB at present, probably things like the mail daemon. It's been useful to have a very slow node, which I can SSH into (after 10 minutes) and kill a chosen process, rather than a dead/OOMed node. But I think the difference is marginal, and perhaps 512MiB would have been a more appropriate size for the partition. (Swappiness is set to 1.)
javitury 9y ago
For many years now I use ram compression instead of swap for desktops/laptops. I particularly like zram but zswap is also great if you you are hitting hardware limits. The difference with swap is that the computer doesn't get unresponsive, it just slows down a bit. And Ram compression still buys some time before OOM killer hits.
coding123 9y ago
My use case for swap: 10 dev environments on a cheap-ass AWS server. Where each environment is about 5 docker images. They are slower, but to give every branch a fully usable test system is pretty awesome. No reason to pay through the butt for RAM for tier-1 dev environments. You can also have a premium dev environment for the develop branch on a different server.
ibiza 9y ago
Count me among the believers in running w/ swap. Here's all it takes to provide Linux with a little swap space: fallocate -l 8G /swapfile chmod 0600 /swapfile mkswap /swapfile swapon /swapfile Add an entry in /etc/fstab & you're done. "This little trick" made all the difference on a compute cluster I managed, where each node contained 96G of RAM. It's much more pleasant to monitor swap usage than the OOMKiller and related kernel parameters.
quotemstr 9y ago
Very few people on this thread read and understood the article. The point isn't working with data sets larger than RAM. The point is making better use of the RAM you do have by taking pages you'll almost never touch and spilling them to disk so that there's more room in RAM for pages you will touch. Banning swap is like making self-storage companies illegal and forcing everyone to hold all possessions in their homes. Sure, you'd be able to get to grandma's half broken kitschy dog coaster that you can't bring yourself to throw away, but you'd also be harder to harder to fit and find your own stuff, the stuff you need all the time. If you find yourself driving to and from the self storage place every day, you probably need a bigger home. But self storage is plenty useful even if you almost never visit it.
- anowlcalledjosh 9y ago
  The issue is that the current OOM killer doesn't support this usage at all. To extend the analogy: what do you do if grandma comes and fills your house with stuff? You need space to work, so you go and drop it off at the self storage place, but what if she just keeps filling your house up? The OOM killer will do absolutely nothing until both your house and the whole self storage place are totally full. By that point, you've spent a huge amount of time just driving to and from self storage, so you haven't had time to do any actual work; it would probably have been better to tell grandma that you don't want any more stuff once she filled up your house for the first time.
  - quotemstr 9y ago
    Well, it doesn't help that when grandma calls and asks whether you have room for more stuff, the Linux kernel responds on your behalf, "Yes, of course I have room. I live in a TARDIS." And you then do all driving to the self-storage facility to maintain the illusion as long as you can. I really don't like overcommit. Anyway, I agree with you that this behavior is annoying, but I think it ought to be possible to fix it (e.g., with memory cgroups or something like Android's lmkd) without giving up on the idea of spilling infrequently-accessed private dirty pages to disk.
    - 9y ago
kartickv 9y ago
How long before desktop OSs manage memory like mobile ones, automatically shutting down background apps that aren't being used, so that the system remains responsive no matter what? Or, worst case, if two tasks that need 8GB each are running on a machine with 8GB memory, kill one, let the other finish, and restart the first one. Or, less ambitious, freeze one, swap it out to disk, let the other finish, and only then resume the frozen app. Desktop OSs are so primitive at memory management, forcing the complexity onto the user.
- vardump 9y ago
  > How long before desktop OSs manage memory like mobile ones, automatically shutting down background apps Once desktop systems and applications support required APIs to handle saving state before being shut down.
  - cdown 9y ago
    > Once desktop systems and applications support required APIs to handle saving state before being shut down. SIGTERM and friends? :-) If your application is just dropping state on the floor as a result of having an intentionally trappable signal being sent to it or its children, that seems like a bug.
black_puppydog 9y ago
Without wanting to impose my method or reasoning here, I run my dev machine without swap, and I'd rather have the same for the cluster machines I access. This is for academic use only. I know how much RAM my machine has, and if I oom, it usually isn't because I tried to squeeze in just a tiny bit too much data, but rather because I made some stupid mistake and keep allocating small chunks of memory very rapidly. On a system with even a moderate amount of swap, this makes everything grind to a halt, and it is usually much faster to just reboot the machine and deal with the problems later in the unlikely event that rebooting actually causes problems.
- viraptor 9y ago
  If you're running a single-(or close to)-purpose machine, then seeing an explicit memory usage limit on the main app could give you even better/faster results.
leoc 9y ago
I'd be more convinced by the argument that swap shouldn't be thought of as slow RAM if the author addressed the fact that it's generally known as 'virtual memory'—and it has been since at least System 370, so it's not simply a later misconception: http://pages.cs.wisc.edu/~stjones/proj/vm_reading/ibmrd2505M.pdf http://pages.cs.wisc.edu/~stjones/proj/vm_reading/ibmrd2505M... . Instead the article just omits the term 'virtual memory' completely, and pretty conspicuously. I also think that a convincing case for swap would have to discuss the concepts of latency, interactivity, and (soft) real-time performance, things that largely weren't to the fore in the salad days of the 370 family or the VAX. Virtual memory is the TCP of local storage.
- JdeBP 9y ago
  That is not the argument. The article actually says, four times over, that it should not be thought of as emergency memory. It's not emergency memory; it's ordinary memory that should see use as part of an everyday memory hierarchy. And if you are going to question the terminology, the elephant in the room that you have missed is calling paging swapping. (-:
  - leoc 9y ago
    Despite the repeated use of the word 'emergency' this is not quite obvious. For example in > many people just see it as a kind of “slow extra memory” for use in emergencies the scare quotes are around 'slow extra memory' not 'emergencies'. Now granted in the last bullet point of the conclusion it affirms that VM is a source of slow memory, but earlier it uses 'memory' where it's referring specifically to RAM, for example > Without swap: Anonymous pages are locked into memory as they have nowhere to go. Really the main reason my original comment was rubbsih is that I took the article far too much as a general discussion of swap when, as it said, it's largely about how much swap to enable on a given Linux system running some already-determined software.
Neil44 9y ago
The system should be tuned so that under excessive pressure from requests it starts to turn requests away before running out of RAM. Having a small ammount of swap will let you get closer to the limit of RAM use without risking OOM killing something and getting the system into an undefined state. Also you can swap a fair bit of the system you don't use and leave it there, giving you more RAM to use for processing requests.
viraptor 9y ago
I feel like the real metric is like to see from swap usage is: how much time did I spend waiting to swap-in a page, how much extra cache pages it gave me, and what's the cache hit ratio. If the big purpose is to allow those extra few pages to be available, then either it's worth doing or not - there should be an objective way to look at this. Unfortunately only the second and third part is easily available. The first... maybe via systemtap?
- cdown 9y ago
  > how much time did I spend waiting to swap-in a page You can do this with eBPF/BCC by using funclatency (https://github.com/iovisor/bcc/blob/master/tools/funclatency.py https://github.com/iovisor/bcc/blob/master/tools/funclatency...) to trace swap related kernel calls. It depends on exactly what you want, but take a look at mm/swap.c and you'll probably find a function which results in the semantics you want.

150 points by c4urself 9y ago

On windows I have found it necessary to disable swap to keep myself efficient. Many times Ive had applications decide to allocate massive amounts of memory and then it leads to my system slowing down with tons of swap activity. In nearly all cases, I didn't want my system to try its best to handle these massive memory requests but rather it should have just killed the offending application. Often in these failure scenarios the swap goes nuts and my computer becomes unresponsive that it takes a long time to even get to kill the bad actor. Thus I disabled swap and I never had these unresponsive issues. I run with 32GB of ram so generally well behaved applications never run into memory issues. Some applications that would cause issues would be too many VirtualBox instances that use more than available memory. A text editor that chocks trying to open a >1GB text file (looking at you, the new JS-based editors.)

maxxxxx 9y ago
Windows is terrible at swapping. As soon as you hit max RAM performance suffers a lot. Even if you never use inactive apps.
- mnx 9y ago
  Yep. When I was using win7 with 2gb of ram, I was basically managing it manually - as soon as memory usage hit 90-95%,I was shutting apps down. If I let it swap, it would often hang completely.

kstenerud 9y ago

So, in other words, if you have enough memory for your workload that you won't run out, there's no benefit to having swap space (i.e. you've wasted money on memory you don't need). But if you DO have swap space, there won't be a performance hit (at least not under Linux) because it will only swap out some rarely used pages and then sit there doing nothing. So, in the general case, it's better to have it and not need it than need it and not have it.

Anderkent 9y ago
> So, in other words, if you have enough memory for your workload that you won't run out, there's no benefit to having swap space (i.e. you've wasted money on memory you don't need). No, that's the opposite. If you have enough memory for your workload that you won't run out, swapping lets you use more memory for disk cache (instead of keeping unaccessed anonymous pages in real ram). Unless by "won't run out" you mean "never have to throw away a disk cache page", which seems very unrealistic.
- clarry 9y ago
  > If you have enough memory for your workload that you won't run out, swapping lets you use more memory for disk cache Except that it doesn't happen in practice, on my systems anyway. If you have plenty of memory, you can keep all your programs in it and as much as the system wants to cache and still not run out. The theory says that swap effectively buys you some memory to spend on more important things (than what the system chooses to page out). So does buying more memory. > Unless by "won't run out" you mean "never have to throw away a disk cache page", which seems very unrealistic. I have an instance of top running on my desktops & laptops all the time. I never see cache using up all of the memory. total used free shared buff/cache available Mem: 7848 1523 5827 53 497 6033
  - Anderkent 9y ago
    Are you talking desktop workflows here? If this is a service, why are you paying for RAM that never gets used? Get a smaller instance.
    - clarry 9y ago
      > Are you talking desktop workflows here? Yes [I explicitly spelled out desktops & laptops] but it's been true on my servers too. > If this is a service, why are you paying for RAM that never gets used? Get a smaller instance. Not all providers are so flexible. I may want a server with more CPU & traffic and some additional disk space. Going there gets me more RAM too. It turns out these services are constrained by the real hardware. If I'm getting a box with enough CPU for my needs, well they are not going to a shop to buy that special box for me, they use what they have and what they have comes with plenty of RAM too.

loeg 9y ago

All of this boils down to "because buying more disk is cheaper than buying more RAM" and "avoid memory contention." The author discusses the situation as if the quantity of RAM is fixed and swap can be added (or not). But that isn't the only possibility — you can also add more RAM (it's just expensive). For the same number of GB of RAM+swap vs just RAM, there is no reason to prefer the option with swap.

Anderkent 9y ago
Sure, but why wouldn't you run some swap with your much bigger ram anyway? In the end the core idea is: sometimes you have anonymous memory that is accessed so rarely that you'd rather have an extra disk cache page. If you assume that the kernel is not paging out memory that you actually use when not under pressure, swap doesn't hurt you.
- clarry 9y ago
  > Sure, but why wouldn't you run some swap with your much bigger ram anyway? If you don't need it, you don't need it. The other question is: how much swap exactly should I have? And why wouldn't I just add that much RAM instead? > In the end the core idea is: sometimes you have anonymous memory that is accessed so rarely that you'd rather have an extra disk cache page. That's the theory. In practice I always have more than enough RAM for all the cached pages the system wants to cache. On my laptop right now (booted today), I have 500MB of cached pages and 5.8 gigabytes of free memory. On my server (booted 499 days ago) I have 700MB of cached pages and 6 gigabytes of free memory. If I were running out of memory [be it for cache or applications], I'd prefer to have more RAM than add swap. Yes, I keep calling it emergency memory. > If you assume that the kernel is not paging out memory that you actually use when not under pressure, swap doesn't hurt you. 1) Bad assumption 2) it doesn't help you either, so why bother? Actually I might have a use for that disk space. In that case the swap just hurts.
  - Anderkent 9y ago
    >If you don't need it, you don't need it. The other question is: how much swap exactly should I have? And why wouldn't I just add that much RAM instead? Because adding RAM costs money, adding swap space is a config setting. It looks to me that your boxes are idle. Sure, if you're not doing any work, it doesn't matter... >1) Bad assumption 2) it doesn't help you either, so why bother? Actually I might have a use for that disk space. In that case the swap just hurts. 1) Is it? Can you source that claim somehow? 2) It does help you, that's the point. It makes your disk reads faster 3) Your answer to needing memory is 'buy more ram', but you 'may have a use for that disk space'? Buy more disk.

nicklaf 9y ago

I have an older Chromebook (c720), which is really quite memory starved (2GB RAM), and have experienced ChromeOS completely frying the SSD simply through prolonged tab-heavy swapping. Now, I've replaced the SSD and installed a non-Google Linux distro, and would like to limit the amount of swapping Firefox can do. I had been planning to simply use cgroups' memory features to limit the amount of memory consumed by Firefox processes, but if I am to understand the article (which I admit I didn't read in full detail), I should also be able to tune swapping to limit the actual amount of swapping that takes place, avoiding a drastic uptick in SSD wear whenever open too many tabs. That, and perhaps a Firefox extension that suspends background tabs in memory (which I've used before with a certain amount of effectiveness in the pre-WebExtension days).

bruce_one 9y ago
Setting the `swappiness` value might be of use to you. Setting a lower value (min is 0, default is 60 iirc, max is 100, so above half) reduces the likelihood of the kernel swapping. The lower the number the fuller ram needs to be before the kernel will start swapping. Hence a low number will mean that less swapping will happen, hence meaning less SSD wear.
- nicklaf 9y ago
  Thanks, I should simply start with that. Along with a hard limit set by cgroups so that browser tabs start being killed in order to stop the swap from being overwhelmed when memory is being used well beyond its capacity. (I'd be interested to know if 'swappiness' already effectively implements the following system-wide, but here goes.) Now that I think of it, it's not so much the quantity of disk storage being consumed by swap, so much as the number of write / delete transactions. One thus wonders if an approach in which the browser somehow favors a small number of tabs to keep in ram, and then dumps the state of the remaining tabs to disk, might be just as effective, but without worrying about the growth of swap. Then, when the user opens a tab that had been saved to disk, if we crudely assume that the memory consumed by open tabs are roughly comparable in memory use, then we can take care of the whole affair of 'swapping' in a single exchange between memory and disk, where we dump the state of the in-memory tab in order to make way for restoring the saved one. (Or did I just reinvent what swappiness does already? From your description of swappiness, I'm inclined to guess the answer is no. This approach strikes me more akin to using the swapfile as a filesystem, and keeping just a small number of tabs paged into RAM.)

whopa 9y ago

This is an informative and well written article, but seems incomplete in this day and age. In public cloud environments, network attached storage is far more prevalent, so the swap story may be different there (I honestly don't know though). Since the author works at Facebook, he probably lacks experience in this regard.

Anderkent 9y ago
Every cloud provider I've worked with (okay, so AWS :P) gives you ephemeral local storage. Obvioulsy you don't swap onto a network drive.
- akerl_ 9y ago
  Even on AWS they're phasing out local storage on new instance types: https://ec2instances.info/ https://ec2instances.info/ (search for ec2 only, but it's the majority of new instance families)
- whopa 9y ago
  Modern AWS instance types are EBS-only.
  - eikenberry 9y ago
    With the exception of the High IO types (I2/I3). They still get it and the newer instances get NVMe SSDs. In other words they are making it a feature of certain types that would benefit from it.
    - _msw_ 9y ago
      For example, F1 instances have NVMe local instance storage. https://aws.amazon.com/ec2/instance-types/f1/ https://aws.amazon.com/ec2/instance-types/f1/
  - Anderkent 9y ago
    Huh. You're right; it seems for the newer instance types only c1.medium and m1.small get swap mounts. That seems like a mistake by aws; but I guess you can a M3 instead of a M5.

avar 9y ago

Even for those who understand this well, it's historically been really hard to coerce the Linux kernel into applying the right swap policies to your application. As the author notes much of this has been improve by cgroups, and there's always been big hammers like mlock(), even with those things it can be hard to prevent memory thrashing in extreme cases. I've seen swap disabled completely by people who understood how it worked as a last result because of that. It's always seemed to me that this was mainly a problem of the kernel configuration being too opaque. Why can't you configure on a system-wide basis that you can use swap e.g. only for anonymous pages and for nothing else? Similarly it would be nice to have a facility for the OOMkiller to call out to userspace (it would need dedicated reserved memory for this) to ask a configured user program "what should I kill?". You might also want to do that when you have 10G left, not 0 bytes.

Hello71 9y ago
Android has this last bit. as explained at https://www.youtube.com/watch?v=ikZ8_mRotT4&t=2145 https://www.youtube.com/watch?v=ikZ8_mRotT4&t=2145 (linked in the article) though, Linux does not have the facilities presently to determine when you have "10G left" in a way that applies across all system configurations.
caf 9y ago
Swap is only used for anonymous pages (well, and dirty private file pages, which are basically the same thing).

wahB4vai 9y ago

I'm a big fan of determinism and service uniformity. Having that rarely used and response time critical function/data/whatever swapped out increases service time variation at best and complicates all worst case response time calculations at least. I understand from the land of JIT compilers, garbage collectors, and oversubscribed everything that this is not much of a substantial concern as these features are already traded away. The swap may be the best case in a bad situation. I would argue along the lines of don't be in a bad situation... I'm looking at you 8 of 16 GB used on cold boot Mac laptops... Looking at you with indignation and rancor Chrome.

caf 9y ago
One of the article's points is that running without swap doesn't necessarily alleviate that. The rarely-used code pages of your rarely-used but response time critical daemon can just as easily be dropped from the page cache and have to be refaulted in from disk, and in fact that's more likely if there isn't swap available to stow the dirty anonymous pages from the cron daemon that wakes up once a day or whatever. The solution for your rarely-used but response time critical daemon is for it to mlock() its critical data and code pages into memory, which works regardless of whether or not you have swap available. (Or, alternatively, use one of the cgroup controllers that the article alludes to, to give the critical daemon and related processes memory unaffected by memory pressure elsewhere in the system).
- AstralStorm 9y ago
  No they cannot. Code that is loaded into memory stays in memory when three is no swap. (Mlock is to prevent swap out or compression - major faults.) The exception would be lazy library loading. BIND_NOW is your friend in this case. Essentially having no swap is similar to having everything mlocked - no major faults can happen except with mmapped files which will just use direct disk IO. If you mean disk caches, when have you seen a multigigabyte executable?
  - caf 9y ago
    Code that is loaded into memory stays in memory when three is no swap. That is not true. In the normal case (absent debugging, JIT, self-modifying code etc), pages of executable code are clean, shared mappings so they do not interact with swap at all. As clean, shared mappings they are eligible to be dropped from the page cache in the same way as other clean file mapped pages. (Your executable code pages essentially are mmapped files. )

deleted 9y ago

[deleted]

slaymaker1907 9y ago

I recently reenabled swap on my Windows machine due to frequent OOM, even with 16GB of RAM while playing Overwatch and browsing on Firefox. It seems like both of these programs allocate vast swaths of memory but then do not actually use that memory very heavily. After I turned swap back on, I did not notice any degradation in performance but my system stability skyrocketed.

wilun 9y ago
Windows has vastly different policies for RAM allocation and commit than Linux. Windows basically does not overcommit while Linux systems not only does it by default but quite depend on it for various loads to work properly. In consequence, the userspace has a tendency to handle RAM differently, but there is no magic: if programs are trying to allocate twice the amount of RAM and then only use half of it, Windows with a swap that can be large enough will work perfectly while without swap the allocation will fail. Under Linux, the situation is less clear: without swap it will succeed (well it depends on the fine details of overcommit that are selected, but you get the idea) although if you really then use all that RAM, the OOM killer will start to more or less "randomly" kill "any" process to cope with the lack of RAM (as a last resort measure though; the caches and buffers are flushed before, etc.)
- Asooka 9y ago
  Windows also has two syscalls - one to reserve memory and one to commit memory. You can reserve+commit at once, but you can also just reserve a chunk of virtual memory that you commit at a later time. Accessing pages that aren't committed is a segfault. So you can say "I might need up to X contiguous bytes of virtual memory" and then commit as you go. IIRC, Windows will let you over-reserve, but not over-commit. Edit: sorry, not two syscals, it's an option to the malloc-equivalent - VirtualAlloc

CoconutPilot 9y ago

Swap was a great idea, but its time is gone. Swap doesn't make sense anymore, hard drives have not scaled and kept up with the improvements in RAM. In the Pentium 1 era EDO RAM maxed out at 256MB/s and hard disk xfer was 10MB/s. Common RAM size was 16MB. In today's era DDR4 maxes out at 32GB/s and hard disk xfer is 500 MB/s. Common RAM size is 16GB. RAM xfer rate has grown is 320x. RAM capacity has grown 100x. Disk xfer rate has grown 50x. Swap is no longer a useful tool.

Dylan16807 9y ago
The purpose of swap is not running things out of it. The purpose is shoveling unused data out of memory. And for that it doesn't need to be particularly fast. Swap has a lot less purpose in a world without memory leaks and extraneous functions. But in practice it's quite good at getting several gigabytes of unnecessary data out of the way, so ram can be used properly. Swap, well-used, should only take up a few percent of the drive's bandwidth.
- lesserknowndan 9y ago
  This reply was a much better explanation than the entire original article, or to summarize “Because memory leaks”.
- tmyklebu 9y ago
  > The purpose of swap is not running things out of it. The purpose is shoveling unused data out of memory. And for that it doesn't need to be particularly fast. You can't detect a priori whether data is "unused." If you guess wrong a few times in a row, you get the familiar pattern where your Linux box is unresponsive to everything and needs to be bounced. If you could detect whether data is rarely used, swap still isn't necessary. Applications can mmap() a file and use that region for "rarely used data" if such is known in advance. Extraneous functions should be backed by the executable in the common case. In the JIT case, they probably won't be JITted anyway. I still think the OOM killer is less intrusive than swapping to disk. It kills some, but not all, of the processes on the machine get killed. The system pretty reliably comes back to life in less time than it takes a human to diagnose the problem and bounce the system. As a bonus, no human needs to get involved.

dboreham 9y ago

We've disabled swap (by not configuring a swap partition) on every server we've deployed since 2009. It's a little irritating to have to manually remove the swap partition from various Linux' "server" default install options even today. Of course this means I'm still installing on bare metal that I own, so...dinosaur.

perlgeek 9y ago

> Under temporary spikes in memory usage > With swap: We’re more resilient to temporary spikes, but in cases of severe memory starvation, the period from memory thrashing beginning to the OOM killer may be prolonged. We have more visibility into the instigators of memory pressure and can act on them more reasonably, and can perform a controlled intervention. Somehow that doesn't resonate with my experience. I tend to remember the cases where I can't even SSH into the box, because the fork in sshd takes minutes, as does spawning the login shell. I'd really like some way to have swap, but still loosen the OOM killer on the biggest memory hog when the system slows down to a crawl. I haven't found that magic configuration yet.

XorNot 9y ago
Swap iotime quotas maybe? I suspect the solution is a lot more complicated though - what I really want is a way to wall off my UI so it stays responsive during swap thrashing and let's me react to the situation.
rfjakob 9y ago
There's a userspace daemon that does that: https://github.com/rfjakob/earlyoom https://github.com/rfjakob/earlyoom (I am the author)
codesnik 9y ago
I remember not being able to SSH on the box too, but this stopped be the case (for me at least) some five years ago - I was able to login to heavy swap trashing boxes without any problem. I thought that something was changed in contemporary linux distros. Either login shell was given much more priority or something like that.
JdeBP 9y ago
Well the article does suggest a mechanism to apply. As for the problem with SSH and login: You might well find that it is not the fork that is the problem. You might well be surprised at how much chaff is run by a login shell, or even by non-login shells. A case in point: I recently reduced the load on a server system that involved lots of SCP activity by noticing that, thanks to RedHat bug #810161, every SCP session even though it was a non-login non-interactive shell was first running a program to enumerate the PCI bus, to fix a problem with a Cirrus graphics adapter card that the machine did not have on a desktop environment that the machine did not have. This driven by /etc/bashrc sourcing /etc/profile.d/* . * https://github.com/FedoraKDE/kde-settings/blob/F-26/etc/profile.d/qt-graphicssystem.sh https://github.com/FedoraKDE/kde-settings/blob/F-26/etc/prof...

cosarara97 9y ago

In my experience, a misbehaving linux system that's out of RAM and has swap to spare will be unusably slow. The process of switching to a tty, logging in, and killing whatever the offending process is can easily take a good 15 minutes. Xorg will just freeze. Oh, and hopefully you know what process it is, else good luck running `top`. Until this is fixed, I'll just keep running my systems with very small amounts of swap (say, 512MB in a system with 16GB of RAM). I'd rather the OOM killer kick in than have to REISUB or hold down the power button. Some benchmarks with regards to the performance claims would be nice.

kzrdude 9y ago
What function does the 0.5GB swap have?
- petecox 9y ago
  I just wish Linux distro installers would make opting out the default option; no, I don't want to swap on my SSD. The last time I installed a distro, I still had to select the manual option for partitioning. With an 8 Gig stick in my NUC, for normal desktop usage it never goes above 3.
  - Karunamon 9y ago
    And even then; they provision an absurd amount of it. I just did a fresh Ubuntu install. On a machine with 32 gigs ram, it creates a 32 gig swap partition by default!
    - KozmoNau7 9y ago
      They probably reason that you want your system to be able to hibernate, plus storage is cheap. I have 8GB of swap for the 8GB in my laptop, for that reason. On my desktop with 16GB, the w 2GB of swap it has is sometimes too little, and everything grinds to a halt.
      - 9y ago

ohazi 9y ago

Does hibernating via a swap file work reasonably well yet? I haven't had a chance to try this out yet, but that's the main reason I still have a swap partition on my laptop.

gerdesj 9y ago
Well done mate - you are the first person to mention this here. It was also only briefly mentioned in the article. Yes, hibernation does work well and it requires swap. Personally, I set a swap partition equal to RAM + 512MB on systems that I want to hibernate on. Linux also supports swap files and this might be handy: https://wiki.debian.org/Hibernation/Hibernate_Without_Swap_Partition https://wiki.debian.org/Hibernation/Hibernate_Without_Swap_P...

Aardwolf 9y ago

In defence against swap on my personal computer: -PCs have a lot of RAM now -When you allocate that much memory it's usually a bug in your own code like a size_t that overflowed. I never saw programs I would actually want to use try to allocate that much -When using swap instead of ram, everything becomes so slow that you're screwed anyway. The UI doesn't even respond fast enough to kill whatever tries to use all that memory. -How common is a situation where you need more memory than your ram size yet less than ram+swap size in a useful way? Usually if something needs a lot, it's really lot (and as mentioned above not desirable) -Added complexity of making extra partition -Added complexity if you want to use full disk encryption -I do the opposite of using disk as ram: I put /tmp in a ramdisk of a few gigs -Disks are slow and fast ssd's are expensive so you would't want to sacrifice their space (maybe if this changes some day...)

mort96 9y ago
Regarding the complexity of making an extra partition: I completely agree, having a separate partition for swap is annoying. Luckily, Linux supports swap files; all my systems (where I had to manually set up swap), I just create a file called /swapfile. I imagine this would solve the full disk encryption complexity too.
IncRnd 9y ago
Exactly! Also, the person who goes from 32G of RAM to 256G of RAM is going to run without swap.
- deleted 9y ago
  [deleted]
- Symbiote 9y ago
  I have 4GiB swapfiles on the cluster nodes I manage, which have 512GiB RAM. It's hardly used, around 300MiB at present, probably things like the mail daemon. It's been useful to have a very slow node, which I can SSH into (after 10 minutes) and kill a chosen process, rather than a dead/OOMed node. But I think the difference is marginal, and perhaps 512MiB would have been a more appropriate size for the partition. (Swappiness is set to 1.)

javitury 9y ago

For many years now I use ram compression instead of swap for desktops/laptops. I particularly like zram but zswap is also great if you you are hitting hardware limits. The difference with swap is that the computer doesn't get unresponsive, it just slows down a bit. And Ram compression still buys some time before OOM killer hits.

coding123 9y ago

My use case for swap: 10 dev environments on a cheap-ass AWS server. Where each environment is about 5 docker images. They are slower, but to give every branch a fully usable test system is pretty awesome. No reason to pay through the butt for RAM for tier-1 dev environments. You can also have a premium dev environment for the develop branch on a different server.

ibiza 9y ago

Count me among the believers in running w/ swap. Here's all it takes to provide Linux with a little swap space: fallocate -l 8G /swapfile chmod 0600 /swapfile mkswap /swapfile swapon /swapfile Add an entry in /etc/fstab & you're done. "This little trick" made all the difference on a compute cluster I managed, where each node contained 96G of RAM. It's much more pleasant to monitor swap usage than the OOMKiller and related kernel parameters.

quotemstr 9y ago

Very few people on this thread read and understood the article. The point isn't working with data sets larger than RAM. The point is making better use of the RAM you do have by taking pages you'll almost never touch and spilling them to disk so that there's more room in RAM for pages you will touch. Banning swap is like making self-storage companies illegal and forcing everyone to hold all possessions in their homes. Sure, you'd be able to get to grandma's half broken kitschy dog coaster that you can't bring yourself to throw away, but you'd also be harder to harder to fit and find your own stuff, the stuff you need all the time. If you find yourself driving to and from the self storage place every day, you probably need a bigger home. But self storage is plenty useful even if you almost never visit it.

anowlcalledjosh 9y ago
The issue is that the current OOM killer doesn't support this usage at all. To extend the analogy: what do you do if grandma comes and fills your house with stuff? You need space to work, so you go and drop it off at the self storage place, but what if she just keeps filling your house up? The OOM killer will do absolutely nothing until both your house and the whole self storage place are totally full. By that point, you've spent a huge amount of time just driving to and from self storage, so you haven't had time to do any actual work; it would probably have been better to tell grandma that you don't want any more stuff once she filled up your house for the first time.
- quotemstr 9y ago
  Well, it doesn't help that when grandma calls and asks whether you have room for more stuff, the Linux kernel responds on your behalf, "Yes, of course I have room. I live in a TARDIS." And you then do all driving to the self-storage facility to maintain the illusion as long as you can. I really don't like overcommit. Anyway, I agree with you that this behavior is annoying, but I think it ought to be possible to fix it (e.g., with memory cgroups or something like Android's lmkd) without giving up on the idea of spilling infrequently-accessed private dirty pages to disk.
  - 9y ago

kartickv 9y ago

How long before desktop OSs manage memory like mobile ones, automatically shutting down background apps that aren't being used, so that the system remains responsive no matter what? Or, worst case, if two tasks that need 8GB each are running on a machine with 8GB memory, kill one, let the other finish, and restart the first one. Or, less ambitious, freeze one, swap it out to disk, let the other finish, and only then resume the frozen app. Desktop OSs are so primitive at memory management, forcing the complexity onto the user.

vardump 9y ago
> How long before desktop OSs manage memory like mobile ones, automatically shutting down background apps Once desktop systems and applications support required APIs to handle saving state before being shut down.
- cdown 9y ago
  > Once desktop systems and applications support required APIs to handle saving state before being shut down. SIGTERM and friends? :-) If your application is just dropping state on the floor as a result of having an intentionally trappable signal being sent to it or its children, that seems like a bug.

black_puppydog 9y ago

Without wanting to impose my method or reasoning here, I run my dev machine without swap, and I'd rather have the same for the cluster machines I access. This is for academic use only. I know how much RAM my machine has, and if I oom, it usually isn't because I tried to squeeze in just a tiny bit too much data, but rather because I made some stupid mistake and keep allocating small chunks of memory very rapidly. On a system with even a moderate amount of swap, this makes everything grind to a halt, and it is usually much faster to just reboot the machine and deal with the problems later in the unlikely event that rebooting actually causes problems.

viraptor 9y ago
If you're running a single-(or close to)-purpose machine, then seeing an explicit memory usage limit on the main app could give you even better/faster results.

leoc 9y ago

I'd be more convinced by the argument that swap shouldn't be thought of as slow RAM if the author addressed the fact that it's generally known as 'virtual memory'—and it has been since at least System 370, so it's not simply a later misconception: http://pages.cs.wisc.edu/~stjones/proj/vm_reading/ibmrd2505M.pdf http://pages.cs.wisc.edu/~stjones/proj/vm_reading/ibmrd2505M... . Instead the article just omits the term 'virtual memory' completely, and pretty conspicuously. I also think that a convincing case for swap would have to discuss the concepts of latency, interactivity, and (soft) real-time performance, things that largely weren't to the fore in the salad days of the 370 family or the VAX. Virtual memory is the TCP of local storage.

JdeBP 9y ago
That is not the argument. The article actually says, four times over, that it should not be thought of as emergency memory. It's not emergency memory; it's ordinary memory that should see use as part of an everyday memory hierarchy. And if you are going to question the terminology, the elephant in the room that you have missed is calling paging swapping. (-:
- leoc 9y ago
  Despite the repeated use of the word 'emergency' this is not quite obvious. For example in > many people just see it as a kind of “slow extra memory” for use in emergencies the scare quotes are around 'slow extra memory' not 'emergencies'. Now granted in the last bullet point of the conclusion it affirms that VM is a source of slow memory, but earlier it uses 'memory' where it's referring specifically to RAM, for example > Without swap: Anonymous pages are locked into memory as they have nowhere to go. Really the main reason my original comment was rubbsih is that I took the article far too much as a general discussion of swap when, as it said, it's largely about how much swap to enable on a given Linux system running some already-determined software.

Neil44 9y ago

The system should be tuned so that under excessive pressure from requests it starts to turn requests away before running out of RAM. Having a small ammount of swap will let you get closer to the limit of RAM use without risking OOM killing something and getting the system into an undefined state. Also you can swap a fair bit of the system you don't use and leave it there, giving you more RAM to use for processing requests.

viraptor 9y ago

I feel like the real metric is like to see from swap usage is: how much time did I spend waiting to swap-in a page, how much extra cache pages it gave me, and what's the cache hit ratio. If the big purpose is to allow those extra few pages to be available, then either it's worth doing or not - there should be an objective way to look at this. Unfortunately only the second and third part is easily available. The first... maybe via systemtap?

cdown 9y ago
> how much time did I spend waiting to swap-in a page You can do this with eBPF/BCC by using funclatency (https://github.com/iovisor/bcc/blob/master/tools/funclatency.py https://github.com/iovisor/bcc/blob/master/tools/funclatency...) to trace swap related kernel calls. It depends on exactly what you want, but take a look at mm/swap.c and you'll probably find a function which results in the semantics you want.