Re: [Qemu-devel] [PATCH V2 1/1] linux-aio: prevent submitting more than MAX_EVENTS

[Paolo Bonzini <pbonzini@redhat.com>]

On 15/07/2016 11:18, Roman Penyaev wrote:
> Those 3 red spikes and a blue hill is what we have to focus on.  The
> blue hill at the right corner of the chart means that almost always the
> ring buffer was observed as full, i.e. qemu_laio_completion_bh() got
> a chance to reap completions not very often, meanwhile completed
> requests stand in the ring buffer for quite a long time which degrades
> the overall performance.
> 
> The results covered by the red line are much better and that can be
> explained by those 3 red spikes, which are almost in the middle of the
> whole distribution, i.e. qemu_laio_completion_bh() is called more often,
> completed requests do not stall, giving fio a chance to submit new fresh
> requests.
> 
> The theoretical fix would be to schedule completion BH just after
> successful io_submit, i.e.:

What about removing the qemu_bh_cancel but keeping the rest of the patch?

I'm also interested in a graph with this patch ("linux-aio: prevent
submitting more than MAX_EVENTS") on top of origin/master.

Thanks for the analysis.  Sometimes a picture _is_ worth a thousand
words, even if it's measuring "only" second-order effects (# of
completions is not what causes the slowdown, but # of completions
affects latency which causes the slowdown).

Paolo