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Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted |
Date: |
Tue, 16 Dec 2014 14:10:34 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 16.12.2014 um 12:28 hat Paolo Bonzini geschrieben:
>
>
> On 16/12/2014 12:07, Kevin Wolf wrote:
> > Am 11.12.2014 um 14:52 hat Paolo Bonzini geschrieben:
> >> Keep a queue of requests that were not submitted; pass them to
> >> the kernel when a completion is reported, unless the queue is
> >> plugged.
> >>
> >> The array of iocbs is rebuilt every time from scratch. This
> >> avoids keeping the iocbs array and list synchronized.
> >>
> >> Signed-off-by: Paolo Bonzini <address@hidden>
> >
> > Just found out that in qemu-img bench, this patch seems to cost about
> > 5-8% for me.
>
> What execution? Queue depth=1?
My usual one:
$ ./qemu-img bench -t none -c 10000000 -n /dev/loop0
Sending 10000000 requests, 4096 bytes each, 64 in parallel
> For me it was noisy but I couldn't see a pessimization, and this patch
> should only add a handful of pointer accesses. Also, does perf point at
> a culprit, and does patch 5 restore some of the performance?
>
> Weird guess: TLB misses from accessing iocbs[0] on the stack (using a
> different coroutine stack every time)? Perf would report that as a
> large cost of this line:
>
> iocbs[len++] = &aiocb->iocb;
No, I can't seem to read much from the perf results. The cost seems to
be spread fairly evenly across ioq_submit(), with the exception of the
instruction after the call to io_submit(). Not sure why the next
instruction always takes so much time (independent of what it is), but
it has been this way before.
I was surprised to see a "rep stos" scoring at 10% in laio_submit(),
apparently io_prep_*() do a memset on the iocb. Not sure if that is
necessary, but again, it has always been this way.
Patch 5 doesn't restore the performance, which makes sense, as qemu-img
only sends single requests.
Kevin
- [Qemu-devel] [PATCH v2 0/5] linux-aio: rewrite and simplify queuing code, Paolo Bonzini, 2014/12/11
- [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted, Paolo Bonzini, 2014/12/11
- Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted, Kevin Wolf, 2014/12/16
- Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted, Paolo Bonzini, 2014/12/16
- Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted,
Kevin Wolf <=
- Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted, Paolo Bonzini, 2014/12/16
- Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted, Paolo Bonzini, 2014/12/16
- Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted, Paolo Bonzini, 2014/12/17
- Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted, Paolo Bonzini, 2014/12/17
[Qemu-devel] [PATCH v2 2/5] linux-aio: track whether the queue is blocked, Paolo Bonzini, 2014/12/11
[Qemu-devel] [PATCH v2 4/5] linux-aio: drop return code from laio_io_unplug and ioq_submit, Paolo Bonzini, 2014/12/11
[Qemu-devel] [PATCH v2 5/5] linux-aio: simplify removal of completed iocbs from the list, Paolo Bonzini, 2014/12/11
[Qemu-devel] [PATCH v2 3/5] linux-aio: rename LaioQueue idx field to "n", Paolo Bonzini, 2014/12/11
Re: [Qemu-devel] [PATCH v2 0/5] linux-aio: rewrite and simplify queuing code, Kevin Wolf, 2014/12/11
Re: [Qemu-devel] [PATCH v2 0/5] linux-aio: rewrite and simplify queuing code, Stefan Hajnoczi, 2014/12/12