4 ms·

io_uring allows the kernel and the user program to communicate purely via shared memory without having to perform a system call, i.e. a context switch to the ke

io_uring allows the kernel and the user program to communicate purely via shared memory without having to perform a system call, i.e. a context switch to the kernel. Do windows completion ports also work that way or do they involve a system call to be performed in order to consume completion events?

ta8645 6y ago
You do still need a single system call (io_uring_submit) to submit each batch of entries in the submission queue. Edit: actually no it's not required in all cases. Thanks for the correction.
- WJW 6y ago
  You can ask the kernel to poll the submission queue and skip io_uring_submit too. (Though you need elevated privileges to do this IIRC)
- ithkuil 6y ago
  I only read about io_uring without yet having a chance of actually using it so take this with a grain of salt: I read that io_uring has two modes, one where you signal via a system call and another that uses memory mapped polling. https://unixism.net/loti/tutorial/sq_poll.html https://unixism.net/loti/tutorial/sq_poll.html states: > Reducing the number of system calls is a major aim for io_uring. To this end, io_uring lets you submit I/O requests without you having to make a single system call. This is done via a special submission queue polling feature that io_uring supports.
- yxhuvud 6y ago
  Submit is not a syscall. io_uring_enter is the only syscall that is used while running a ring. That one may submit, wait or both at the same time. Strictly speaking it isn't necessary but to avoid it you require elevated privileges.
  - frevib 6y ago
    From Linux 5.10 you only need CAP_SYS_NICE to perform SQPOLL: https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-files_struct&id=03a54c18518d196693217aa9f683369eacb01daf https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-fi...
    - yxhuvud 6y ago
      Yes, hence "elevated" and not "root". It is still higher than default, right?
- junon 6y ago
  Not with SQPOLL. You can eliminate all syscalls with SQPOLL.
Matthias247 6y ago
Windows registered IO (RIO) does imho the same (https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-r2-and-2012/hh997032(v=ws.11) https://docs.microsoft.com/en-us/previous-versions/windows/i...). When enqueuing reads/writes with RIO there at least exist flags to specify that the kernel should not immediately be woken up, and thereby to batch syscalls as with io_uring.

ta8645 6y ago

You do still need a single system call (io_uring_submit) to submit each batch of entries in the submission queue. Edit: actually no it's not required in all cases. Thanks for the correction.

WJW 6y ago
You can ask the kernel to poll the submission queue and skip io_uring_submit too. (Though you need elevated privileges to do this IIRC)
ithkuil 6y ago
I only read about io_uring without yet having a chance of actually using it so take this with a grain of salt: I read that io_uring has two modes, one where you signal via a system call and another that uses memory mapped polling. https://unixism.net/loti/tutorial/sq_poll.html https://unixism.net/loti/tutorial/sq_poll.html states: > Reducing the number of system calls is a major aim for io_uring. To this end, io_uring lets you submit I/O requests without you having to make a single system call. This is done via a special submission queue polling feature that io_uring supports.
yxhuvud 6y ago
Submit is not a syscall. io_uring_enter is the only syscall that is used while running a ring. That one may submit, wait or both at the same time. Strictly speaking it isn't necessary but to avoid it you require elevated privileges.
- frevib 6y ago
  From Linux 5.10 you only need CAP_SYS_NICE to perform SQPOLL: https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-files_struct&id=03a54c18518d196693217aa9f683369eacb01daf https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-fi...
  - yxhuvud 6y ago
    Yes, hence "elevated" and not "root". It is still higher than default, right?
junon 6y ago
Not with SQPOLL. You can eliminate all syscalls with SQPOLL.

Matthias247 6y ago

Windows registered IO (RIO) does imho the same (https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-r2-and-2012/hh997032(v=ws.11) https://docs.microsoft.com/en-us/previous-versions/windows/i...). When enqueuing reads/writes with RIO there at least exist flags to specify that the kernel should not immediately be woken up, and thereby to batch syscalls as with io_uring.