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Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode

From: Karl Rister
Subject: Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode
Date: Mon, 14 Nov 2016 08:52:21 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 11/14/2016 07:53 AM, Fam Zheng wrote:
> On Fri, 11/11 13:59, Karl Rister wrote:
>> Stefan
>> I ran some quick tests with your patches and got some pretty good gains,
>> but also some seemingly odd behavior.
>> These results are for a 5 minute test doing sequential 4KB requests from
>> fio using O_DIRECT, libaio, and IO depth of 1.  The requests are
>> performed directly against the virtio-blk device (no filesystem) which
>> is backed by a 400GB NVme card.
>>                unset    31,383
>>                    1    46,860
>>                    2    46,440
>>                    4    35,246
>>                    8    34,973
>>                   16    46,794
>>                   32    46,729
>>                   64    35,520
>>                  128    45,902
> For sequential read with ioq=1, each request takes >20000ns under 45,000 IOPs.
> Isn't a poll time of 128ns a mismatching order of magnitude? Have you tried
> larger values? Not criticizing, just trying to understand how it workd.

Not yet, I was just trying to get something out as quick as I could
(while juggling this with some other stuff...).  Frankly I was a bit
surprised that the low values made such an impact and then got
distracted by the behaviors of 4, 8, and 64.

> Also, do you happen to have numbers for unpatched QEMU (just to confirm that
> "unset" case doesn't cause regression) and baremetal for comparison?

I didn't run this exact test on the same qemu.git master changeset
unpatched.  I did however previously try it against the v2.7.0 tag and
got somewhere around 27.5K IOPs.  My original intention was to apply the
patches to v2.7.0 but it wouldn't build.

We have done a lot of testing and tracing on the qemu-rhev package and
27K IOPs is about what we see there (with tracing disabled).

Given the patch discussions I saw I was mainly trying to get a sniff
test out and then do a more complete workup with whatever updates are made.

I should probably note that there are a lot of pinning optimizations
made here to assist in our tracing efforts which also result in improved
performance.  Ultimately, in a proper evaluation of these patches most
of that will be removed so the behavior may change somewhat.

> Fam

Karl Rister <address@hidden>

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