Re: [Devel] [PATCH hci-8.1 6/7] drivers/md/dm-qcow2: declare DM_TARGET_NOWAIT

[no-reply@sashiko.virtuozzo.com]

> dm-qcow2 is a request-based target: REQ_NOWAIT handling is done by blk-mq
> (tag allocation) before a request reaches us, and clone_and_map_rq()
> never blocks. Still, the target does not declare DM_TARGET_NOWAIT, so
> dm_table_set_restrictions() clears BLK_FEAT_NOWAIT on the mapped device
> and the block device file gets no FMODE_NOWAIT.
> 
> As a result io_uring is unable to issue I/O in nowait mode and punts
> *every* request to io-wq worker threads: on 4k randwrite (io_uring,
> qd=64, 4 jobs) io-wq threads burn ~24% of all cpus doing nothing but a
> thread handoff.
> 
>                      before            after
>   qd=64 nj=4 IOPS    1034K (11% idle)  892K (30% idle)
>   qd=1  nj=1 lat     8us               6us
> 
> The lower nj=4 number is because io-wq threads were providing "free"
> parallelism for the single-cpu submitters; the next patch gets it back
> without burning the cpus.
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-95494
> Feature: dm-qcow2: block device over QCOW2 files driver
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.vzint.dev/#/patchset/20261029140346.465768-1-andrey.zhadchenko@virtuozzo.com?part=6


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