linux-kernel - Re: [PATCH] blk-mq: allow hardware queue to get more tag while sharing a tag set

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Message-ID: <ade72519-5e16-1cc5-9a77-cb9ead42035e@acm.org>
Date:   Sat, 31 Jul 2021 10:15:14 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Yu Kuai <yukuai3@...wei.com>, axboe@...nel.dk, ming.lei@...hat.com
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH] blk-mq: allow hardware queue to get more tag while
 sharing a tag set

On 7/11/21 8:18 PM, Yu Kuai wrote:
> If there are multiple active queues while sharing a tag set, it's not
> necessary to limit the available tags as same share for each active queue
> if no one ever failed to get driver tag. And fall back to same share if
> someone do failed to get driver tag.
> 
> This modification will be beneficial if total queue_depth of disks
> on the same host is less than total tags.

This patch adds new atomic operations in the hot path and hence probably
has a negative performance impact. What is the performance impact of
this patch for e.g. null_blk when submitting I/O from all CPU cores?

Thanks,

Bart.