linux-kernel - Re: [PATCH] block/mq-deadline: Speed up the dispatch of low-priority requests

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Date:   Thu, 26 Aug 2021 12:13:39 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Bart Van Assche <bvanassche@....org>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Cc:     Damien Le Moal <damien.lemoal@....com>
Subject: Re: [PATCH] block/mq-deadline: Speed up the dispatch of low-priority
 requests

On 8/26/21 12:09 PM, Bart Van Assche wrote:
> On 8/26/21 7:40 AM, Zhen Lei wrote:
>> lock protection needs to be added only in dd_finish_request(), which
>> is unlikely to cause significant performance side effects.
> 
> Not sure the above is correct. Every new atomic instruction has a
> measurable performance overhead. But I guess in this case that
> overhead is smaller than the time needed to sum 128 per-CPU variables.

perpcu counters only really work, if the summing is not in a hot path,
or if the summing is just some "not zero" thing instead of a full sum.
They just don't scale at all for even moderately sized systems.

>> Tested on my 128-core board with two ssd disks.
>> fio bs=4k rw=read iodepth=128 cpus_allowed=0-95 <others>
>> Before:
>> [183K/0/0 iops]
>> [172K/0/0 iops]
>>
>> After:
>> [258K/0/0 iops]
>> [258K/0/0 iops]
> 
> Nice work!
> 
>> Fixes: fb926032b320 ("block/mq-deadline: Prioritize high-priority requests")
> 
> Shouldn't the Fixes: tag be used only for patches that modify
> functionality? I'm not sure it is appropriate to use this tag for
> performance improvements.

For a regression this big, I think it's the right thing. Anyone that may
backport the original commit definitely should also get the followup
fix. This isn't just a performance improvement, it's fixing a big
performance regression.

-- 
Jens Axboe