Re: [Qemu-devel] [PATCH v2 1/5] linux-aio: queue requests that cannot be submitted

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/12/2014 14:10, Kevin Wolf wrote:
> Am 16.12.2014 um 12:28 hat Paolo Bonzini geschrieben:
>>
>>
>> On 16/12/2014 12:07, Kevin Wolf wrote:
>>> Am 11.12.2014 um 14:52 hat Paolo Bonzini geschrieben:
>>>> Keep a queue of requests that were not submitted; pass them to
>>>> the kernel when a completion is reported, unless the queue is
>>>> plugged.
>>>>
>>>> The array of iocbs is rebuilt every time from scratch.  This
>>>> avoids keeping the iocbs array and list synchronized.
>>>>
>>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>>
>>> Just found out that in qemu-img bench, this patch seems to cost about
>>> 5-8% for me.
>>
>> What execution?  Queue depth=1?
> 
> My usual one:
> 
> $ ./qemu-img bench -t none -c 10000000 -n /dev/loop0
> Sending 10000000 requests, 4096 bytes each, 64 in parallel

I could reproduce this very well on a random OS image that I had around.
 This is raw over XFS over dm-crypt, and the image is about 75% sparse
(8.2G used over 35G).  I only get 1-2%, but still it's visible.

However I can hardly reproduce it when using a partition directly:

         old    new
mean     9.9565 9.9636      (+0.07%)
stddev   0.0405 0.0537
min      9.871  9.867
median   9.973  9.971
max      10.01  10.053
count    20     20

I haven't tried removing layers (e.g. fully-allocated XFS image without
dm-crypt).

Paolo