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Re: [Qemu-devel] [Qemu-block] [PATCH] linux-aio: Cancel BH if not needed
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH] linux-aio: Cancel BH if not needed |
Date: |
Mon, 20 Jun 2016 11:34:02 +0100 |
User-agent: |
Mutt/1.6.1 (2016-04-27) |
On Fri, Jun 17, 2016 at 12:24:11PM +0200, Kevin Wolf wrote:
> Am 17.06.2016 um 12:13 hat Stefan Hajnoczi geschrieben:
> > On Wed, Jun 15, 2016 at 01:16:42PM +0200, Kevin Wolf wrote:
> > > linux-aio uses a BH in order to make sure that the remaining completions
> > > are processed even in nested event loops of completion callbacks in
> > > order to avoid deadlocks.
> > >
> > > There is no need, however, to have the BH overhead for the first call
> > > into qemu_laio_completion_bh() or after all pending completions have
> > > already been processed. Therefore, this patch calls directly into
> > > qemu_laio_completion_bh() in qemu_laio_completion_cb() and cancels
> > > the BH after qemu_laio_completion_bh() has processed all pending
> > > completions.
> > >
> > > Signed-off-by: Kevin Wolf <address@hidden>
> > > ---
> > > block/linux-aio.c | 4 +++-
> > > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > I ran 4 x iodepth=16 random 4KB read I/O benchmarks. There might be an
> > improvement but it's within the error margin. My benchmarking setup can
> > be noisy...
> >
> > Anyway, this patch doesn't hurt performance. Guest and host are RHEL 7.2.
>
> Thanks for confirming!
>
> > $ ./analyze.py runs/
> > Name IOPS Error
> > linux-aio-bh-optimizations-ccb9dc1 12942616.0 ± 16.83%
> > linux-aio-bh-optimizations-ccb9dc1-2 13833110.4 ± 4.74%
> > linux-aio-bh-optimizations-off-23b0d9f 13303981.4 ± 2.21%
>
> What are these three commits? Is the first one with your virtio-blk
> changes and this patch, the second only this patch and the third one
> the baseline?
The git trees are from qemu.git/master with no out-of-tree patches (my
stuff isn't there).
1. linux-aio-bh-optimizations-ccb9dc1 12942616.0 ± 16.83%
This is your "linux-aio: Cancel BH if not needed" in qemu.git/master.
2. linux-aio-bh-optimizations-ccb9dc1-2 13833110.4 ± 4.74%
I wanted to try again because the 16.83% error margin is very high and
probably due to noise. This is just a re-run of #1.
3. linux-aio-bh-optimizations-off-23b0d9f 13303981.4 ± 2.21%
This is the commit before your "linux-aio: Cancel BH if not needed"
(ccb9dc1^ == 23b0d9f).
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