Re: QEMU 5.0 virtio-blk performance regression with high queue depths

[Stefan Hajnoczi]

On Wed, Sep 16, 2020 at 5:43 PM Denis V. Lunev <den@virtuozzo.com> wrote:
> On 9/16/20 5:07 PM, Denis V. Lunev wrote:
> > I will make a check today.
> >
> > Talking about our performance measurements, we have not
> > seen ANY performance degradation, especially 30-40%.
> > This looking quite strange to me.
> >
> > Though there is quite important difference. We are always
> > using O_DIRECT and 'native' AIO engine.
> >
> > Den
>
> I have put my hands into this and it looks like you are right. There is
> a difference. It is not as significant for me as in your case, but I observe
> stable around 10% difference with 128 vs 256 queue size.
>
> I have checked with:
> - QEMU 5.1
> - Fedora 31 in guest
> - qcow2 (64k, 1Mb) and raw image on host
> - nocache and both threaded/native IO modes
>
> The test was run on Thinkpad Carbon X1 gen 6 laptop.
>
> For the reference, I have seen 330k IOPS for read
> at max which is looking awesome for native and 220k
> IOPS for threads.

Thanks for confirming! Reverting the commit is unattractive since it
does improve performance in some cases.

It would be good to understand the root cause so the regression can be
fixed without reducing queue-size again.

Do you have time to investigate?

Thanks,
Stefan