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

[Kevin Wolf]

Am 13.07.2016 um 13:33 hat Roman Penyaev geschrieben:
> Just to be sure that we are on the same page:
> 
> 1. We have this commit "linux-aio: Cancel BH if not needed" which
> 
>    a) introduces performance regression on my fio workloads on the
>       following config: "iothread=1, VCPU=8, MQ=8". Performance
>       dropped from 1878MB/s to 1606MB/s with Stefan's fix, that is
>       ~14%.

Do we already understand why the performance regresses with the patch?
As long as we don't, everything we do is just guesswork.

Kevin

>    b) reproduces IO hang, because of in-flights > MAX_EVENTS.
> 
>  So probably this commit should be reverted because of a) not b).
> 
> 2. Stefan has fix for 1.b) issue repeating io_getevents(), which
>    is obviously an excess for generic cases where MQ=1 (queue depth
>    for virtio_blk is also set to 128 i.e. equal to MAX_EVENTS on
>    QEMU side).
> 
> 3. The current patch also aims to fix 1.b) issue restricting number
>    of in-flights.
> 
> 
> Reverting 1. will fix all the problems, without any need to apply
> 2. or 3.  The most lazy variant.
> 
> Restricting in-flights is also a step forward, since submitting
> till -EAGAIN is also possible, but leads (as we already know) to
> IO hang on specific loads and conditions.
> 
> But 2. and 3. are mutual exclusive and should not be applied
> together.
> 
> So we have several alternatives and a choice what to follow.
> 
> --
> Roman