Re: [PATCH v2 3/3] qcow2: handle_dependencies(): relax conflict detection

[Hanna Reitz]

On 24.08.21 12:15, Vladimir Sementsov-Ogievskiy wrote:
> There is no conflict and no dependency if we have parallel writes to
> different subclusters of one cluster when the cluster itself is already
> 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%
> ------------------  ---------  ---------
>
> 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. Filter out second and third write
> offsets to cover both possible outputs.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/qcow2-cluster.c      | 11 +++++++++++
>   tests/qemu-iotests/271     |  5 ++++-
>   tests/qemu-iotests/271.out |  4 ++--
>   3 files changed, 17 insertions(+), 3 deletions(-)

[...]

> diff --git a/tests/qemu-iotests/271 b/tests/qemu-iotests/271
> index 599b849cc6..d9d391955e 100755
> --- a/tests/qemu-iotests/271
> +++ b/tests/qemu-iotests/271
> @@ -893,7 +893,10 @@ EOF
>   }
>   
>   _make_test_img -o extended_l2=on 1M
> -_concurrent_io     | $QEMU_IO | _filter_qemu_io
> +# Second an third writes in _concurrent_io() are independent and may finish in

s/ an / and /

With that fixed:

Reviewed-by: Hanna Reitz <hreitz@redhat.com>

> +# different order. So, filter offset out to match both possible variants.
> +_concurrent_io     | $QEMU_IO | _filter_qemu_io | \
> +    $SED -e 's/\(20480\|40960\)/OFFSET/'
>   _concurrent_verify | $QEMU_IO | _filter_qemu_io
>   
>   # success, all done