7 ms·

I have a lot of personal success stories with SSDs but here's my current favorite. A few months ago I had to help one of our scientists (the company is called

I have a lot of personal success stories with SSDs but here's my current favorite. A few months ago I had to help one of our scientists (the company is called 5AM Solutions.. they the awesome) run a bioinformatic job written in Perl and R. As it turned out, for long stretches of the processing the job required around 20 GB of memory. The one server that had all the required dependencies installed had only 8 GB at the time. When I let the job run the first time, it started to page out memory to hard disk. The job ran for about four days, was only about 25% complete and during that time frame the server was un-useable for any other functions. Pretty much everything came to grinding halt. Between that first run and the time our new RAM would be installed, just for grins, I gave the system 30 GB of swap space on the locally attached SSD. With that configuration the job finished in 19 hours and during that time the server was still responsive of other tasks. When we finally added the appropriate amount of physical RAM the job took only 15 hours to complete. It is the first time I have ever seen virtual memory be useful.

scott_s 15y ago
"Virtual memory" is not a synonym for swap space: http://en.wikipedia.org/wiki/Virtual_memory http://en.wikipedia.org/wiki/Virtual_memory Virtual memory is what lets us write programs pretending that we own the entire address space, and it is very useful. Swapping pages to disk, though, has been useful for a very long time. Yes, once your high-performance application starts swapping all the time, your performance is going to suffer by several orders of magnitude. But occasionally swapping pages in and out of disk is part of what makes modern operating systems useful. You left a large PowerPoint presentation open for several days, but never got around to working on it? Not a problem, since if the OS needs that memory, it will just swap out the pages. Without that ability, the OS would need to go around killing processes. (Which it will do if it has to, but it's a rare event because it can swap out pages.)
- cowmixtoo 15y ago
  My point was that my application needed 166% more phyical memory than what I had at the time. Paging out to a normal "disk" was not even an option, the performance ranged incredibly horrible to completely unusable. Paging out to SSD was not only usable, it was only a percentage slower than 'real' RAM. I think that's amazing. Having the option to use SSD as memory in a pinch is a great 'win'.
- cowmixtoo 15y ago
  Hmm... I still think its valid to interchangeably use "Virtual memory" and swap space. Swap space is just where your "virtual memory" lives, right?
  - fennokin 15y ago
    It's where your "virtual memory lives" in the event that you run out of physical memory. They are not interchangeable.
  - Locke1689 15y ago
    Assuming you have demand paging, your virtual memory doesn't "live" anywhere.
  - scott_s 15y ago
    On modern systems, there are two kinds of addresses. "Virtual" addresses and physical addresses. Virtual addresses are tracked by the operating system, and they can span the entirety of the addressable address space. So, on a 32-bit system that isn't playing any high-memory tricks, that's 0 - 2^32, or 0 - 4 GB. But your system may not have 4 GB. So the operating system has a data structure called a page table that has the virtual to physical mapping for each process. The processor accesses this table (it caches it in something called a TLB) so that it can convert the virtual address to the physical address. An example using small numbers. Your program has a pointer to data. That pointer may have value 800. Let's assume that the amount of memory on your system is only between 0 - 400. So the processor has to convert the value 800 to a value between 0 - 400. It's the operating system's job to maintain that valid mapping. Why does this matter, and why is it so tied up with paging to and from disk? Let's say the OS pages out the page containing that data. Then, later, it's paged back in, but in a different physical location in memory. Your program still has the pointer value 800, but your program still works correctly because the operating system keeps track of where in physical memory 800, for your process, maps to. People in the Windows world often say "virtual memory" when they mean "swap space" because Windows would call the amount of swap space "virtual memory size." But virtual memory is the technique described above. Read the Wikipedia entry from above, or read an operating systems textbook for a full discussion of it.

scott_s 15y ago

"Virtual memory" is not a synonym for swap space: http://en.wikipedia.org/wiki/Virtual_memory http://en.wikipedia.org/wiki/Virtual_memory Virtual memory is what lets us write programs pretending that we own the entire address space, and it is very useful. Swapping pages to disk, though, has been useful for a very long time. Yes, once your high-performance application starts swapping all the time, your performance is going to suffer by several orders of magnitude. But occasionally swapping pages in and out of disk is part of what makes modern operating systems useful. You left a large PowerPoint presentation open for several days, but never got around to working on it? Not a problem, since if the OS needs that memory, it will just swap out the pages. Without that ability, the OS would need to go around killing processes. (Which it will do if it has to, but it's a rare event because it can swap out pages.)

cowmixtoo 15y ago
My point was that my application needed 166% more phyical memory than what I had at the time. Paging out to a normal "disk" was not even an option, the performance ranged incredibly horrible to completely unusable. Paging out to SSD was not only usable, it was only a percentage slower than 'real' RAM. I think that's amazing. Having the option to use SSD as memory in a pinch is a great 'win'.
cowmixtoo 15y ago
Hmm... I still think its valid to interchangeably use "Virtual memory" and swap space. Swap space is just where your "virtual memory" lives, right?
- fennokin 15y ago
  It's where your "virtual memory lives" in the event that you run out of physical memory. They are not interchangeable.
- Locke1689 15y ago
  Assuming you have demand paging, your virtual memory doesn't "live" anywhere.
- scott_s 15y ago
  On modern systems, there are two kinds of addresses. "Virtual" addresses and physical addresses. Virtual addresses are tracked by the operating system, and they can span the entirety of the addressable address space. So, on a 32-bit system that isn't playing any high-memory tricks, that's 0 - 2^32, or 0 - 4 GB. But your system may not have 4 GB. So the operating system has a data structure called a page table that has the virtual to physical mapping for each process. The processor accesses this table (it caches it in something called a TLB) so that it can convert the virtual address to the physical address. An example using small numbers. Your program has a pointer to data. That pointer may have value 800. Let's assume that the amount of memory on your system is only between 0 - 400. So the processor has to convert the value 800 to a value between 0 - 400. It's the operating system's job to maintain that valid mapping. Why does this matter, and why is it so tied up with paging to and from disk? Let's say the OS pages out the page containing that data. Then, later, it's paged back in, but in a different physical location in memory. Your program still has the pointer value 800, but your program still works correctly because the operating system keeps track of where in physical memory 800, for your process, maps to. People in the Windows world often say "virtual memory" when they mean "swap space" because Windows would call the amount of swap space "virtual memory size." But virtual memory is the technique described above. Read the Wikipedia entry from above, or read an operating systems textbook for a full discussion of it.