Re: Fio regression caused by f9fc8932b11f3bcf2a2626f567cb6fdd36a33a94

[Daniel P . Berrangé]

On Thu, May 05, 2022 at 11:09:08AM +0100, Stefan Hajnoczi wrote:
> On Tue, May 03, 2022 at 09:43:15AM +0200, Lukáš Doktor wrote:
> > Hello Mike, Paolo, others,
> > 
> > in my perf pipeline I noticed a regression bisected to the 
> > f9fc8932b11f3bcf2a2626f567cb6fdd36a33a94 - "thread-posix: remove the posix 
> > semaphore support" commit and I'd like to ask you to verify it might have 
> > caused that and eventually consider fixing it. The regression is visible, 
> > reproducible and clearly bisectable to this commit with the following 2 
> > scenarios:
> 
> I can't parse the commit message for
> f9fc8932b11f3bcf2a2626f567cb6fdd36a33a94, so it's not 100% clear to me
> why it was necessary to remove sem_*() calls.
> 
> util/thread-pool.c uses qemu_sem_*() to notify worker threads when work
> becomes available. It makes sense that this operation is
> performance-critical and that's why the benchmark regressed.

Doh, I questioned whether the change would have a performance impact,
and it wasn't thought to be used in perf critical places

   https://www.mail-archive.com/qemu-devel@nongnu.org/msg870737.html

With regards,
Daniel
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