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
