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Re: [Qemu-devel] [Qemu-block] [PATCH v4 8/8] linux-aio: share one LinuxA
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH v4 8/8] linux-aio: share one LinuxAioState within an AioContext |
Date: |
Tue, 10 May 2016 11:40:33 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 10.05.2016 um 11:30 hat Stefan Hajnoczi geschrieben:
> On Mon, May 09, 2016 at 06:31:44PM +0200, Paolo Bonzini wrote:
> > On 19/04/2016 11:09, Stefan Hajnoczi wrote:
> > >> > This has better performance because it executes fewer system calls
> > >> > and does not use a bottom half per disk.
> > > Each aio_context_t is initialized for 128 in-flight requests in
> > > laio_init().
> > >
> > > Will it be possible to hit the limit now that all drives share the same
> > > aio_context_t?
> >
> > It was also possible before, because the virtqueue can be bigger than
> > 128 items; that's why there is logic to submit I/O requests after an
> > io_get_events. As usual when the answer seems trivial, am I
> > misunderstanding your question?
>
> I'm concerned about a performance regression rather than correctness.
>
> But looking at linux-aio.c there *is* a correctness problem:
>
> static void ioq_submit(struct qemu_laio_state *s)
> {
> int ret, len;
> struct qemu_laiocb *aiocb;
> struct iocb *iocbs[MAX_QUEUED_IO];
> QSIMPLEQ_HEAD(, qemu_laiocb) completed;
>
> do {
> len = 0;
> QSIMPLEQ_FOREACH(aiocb, &s->io_q.pending, next) {
> iocbs[len++] = &aiocb->iocb;
> if (len == MAX_QUEUED_IO) {
> break;
> }
> }
>
> ret = io_submit(s->ctx, len, iocbs);
> if (ret == -EAGAIN) {
> break;
> }
> if (ret < 0) {
> abort();
> }
>
> s->io_q.n -= ret;
> aiocb = container_of(iocbs[ret - 1], struct qemu_laiocb, iocb);
> QSIMPLEQ_SPLIT_AFTER(&s->io_q.pending, aiocb, next, &completed);
> } while (ret == len && !QSIMPLEQ_EMPTY(&s->io_q.pending));
> s->io_q.blocked = (s->io_q.n > 0);
> }
>
> io_submit() may have submitted some of the requests when -EAGAIN is
> returned. QEMU gets no indication of which requests were submitted.
My understanding (which is based on the manpage rather than code) is
that -EAGAIN is only returned if no request could be submitted. In other
cases, the number of submitted requests is returned (similar to how
short reads work).
> It may be possible to dig around in the s->ctx rings to find out or we
> need to keep track of the number of in-flight requests so we can
> prevent ever hitting EAGAIN.
>
> ioq_submit() pretends that no requests were submitted on -EAGAIN and
> will submit them again next time. This could result in double
> completions.
Did you check in the code that this can happen?
> Regarding performance, I'm thinking about a guest with 8 disks (queue
> depth 32). The worst case is when the guest submits 32 requests at once
> but the Linux AIO event limit has already been reached. Then the disk
> is starved until other disks' requests complete.
Sounds like a valid concern.
Kevin
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