5 ms·
Why would you use uring to read from an mmap? Couldn't you just memcpy?
by 0c3ca83 17d ago
Why would you use uring to read from an mmap? Couldn't you just memcpy?
- jeffbee 17d agoMaybe they meant using IORING_OP_MADVISE with MADV_WILLNEED to bring ranges into memory?
- marginalia_nu 17d agoThough at that point you can just use FADV_WILLNEED.
- jeffbee 17d agoI'm just steelmanning here ¯\_(ツ)_/¯
- adrian_b 15d agoThe Linux-specific MADV_POPULATE_READ is much more reliable than MADV_WILLNEED if you want to implement read-ahead for a memory-mapped file. In my opinion, the POSIX advices specified for madvise are useless or even dangerous. On Linux, for precise control of memory-mapped files one should use only these 4 Linux-specific advices: MADV_COLD, MADV_PAGEOUT, MADV_POPULATE_READ & MADV_POPULATE_WRITE. These should be used within io_uring, so that they will be executed asynchronously. These have a well-documented meaning and using them carefully should be sufficient to reach optimum performance with mmap.
- deleted 17d ago[deleted]
- Asmod4n 17d agoTo be more precise: i use that map to send assets out directly to clients from a zip file. Its a new web server i am building and its the fastest way i could find out. Just switching from epoll to liburing made the server ~45% faster too, its ridiculous. It can serve 10 gigabyte per second with a single thread, or around 10 million responses per second with h2 and 32 multiplexed requests. I had to write a new http load generator for that since i couldn't find one which could generate enough load to saturate my server or be fast enough to withstand it.
- withinboredom 17d ago10 gb per second is pretty slow for a disk. You should be seeing much higher than that.
- marginalia_nu 16d ago??? The only configuration that will allow disk reads at 10 GB/s is if you're using PCIe 5.0. PCIe 4.0 or lower, and SATA will not drop out long before that. He's very clearly hauling data straight from the page cache.
- 0c3ca83 16d ago...I'm still confused why you wouldn't use memcpy?
- Asmod4n 15d agoHow can one memcpy from a fd to a socket? I don’t touch the bytes in userspace, I just tell the kernel to send them out.
- 0c3ca83 13d agoSo why would you mmap?
- Asmod4n 12d agoto not let the bytes i forward enter userspace, i serve from the page cache for the asset path, directly from a zip file. i open the file, mmap it, close it and tell io_uring_prep_send which bytes from the mapping to send, this also saves me from a possible SIGBUS cause the access to the mmap happen inside the kernel, and when a SIGBUS would happen in userspace the kernel just reports a shorter send in cqe->res