Re: [PATCH 3/3] qcow2: handle_dependencies(): relax conflict detection

[Eric Blake]

On Sat, Jul 24, 2021 at 04:38:46PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> There is no conflict and no dependency if we have parallel writes to
> different subclusters of one cluster when cluster itself is already

when the cluster itself

> allocated. So, relax extra dependency.
> 
> Measure performance:
> First, prepare build/qemu-img-old and build/qemu-img-new images.
> 
> cd scripts/simplebench
> ./img_bench_templater.py
> 
> Paste the following to stdin of running script:
> 
> qemu_img=../../build/qemu-img-{old|new}
> $qemu_img create -f qcow2 -o extended_l2=on /ssd/x.qcow2 1G
> $qemu_img bench -c 100000 -d 8 [-s 2K|-s 2K -o 512|-s $((1024*2+512))] \
>         -w -t none -n /ssd/x.qcow2
> 
> The result:
> 
> All results are in seconds
> 
> ------------------  ---------  ---------
>                     old        new
> -s 2K               6.7 ± 15%  6.2 ± 12%
>                                  -7%
> -s 2K -o 512        13 ± 3%    11 ± 5%
>                                  -16%
> -s $((1024*2+512))  9.5 ± 4%   8.4
>                                  -12%
> ------------------  ---------  ---------

Cool improvement.

> 
> So small writes are more independent now and that helps to keep deeper
> io queue which improves performance.
> 
> 271 iotest output becomes racy for three allocation in one cluster.
> Second and third writes may finish in different order. Second and
> third requests don't depend on each other any more. Still they both
> depend on first request anyway. Keep only one for consistent output.

Interesting fallout.  Yes, it looks like the test is still robust
enough without the extra request.

> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/qcow2-cluster.c      | 11 +++++++++++
>  tests/qemu-iotests/271     |  4 +---
>  tests/qemu-iotests/271.out |  2 --
>  3 files changed, 12 insertions(+), 5 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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