linux-kernel - Re: [RFC] block: Change the granularity of io ticks from ms to ns

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Date:   Wed, 7 Dec 2022 21:09:08 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Ming Lei <ming.lei@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     Gulam Mohamed <gulam.mohamed@...cle.com>,
        linux-block@...r.kernel.org, axboe@...nel.dk,
        philipp.reisner@...bit.com, lars.ellenberg@...bit.com,
        christoph.boehmwalder@...bit.com, minchan@...nel.org,
        ngupta@...are.org, senozhatsky@...omium.org, colyli@...e.de,
        kent.overstreet@...il.com, agk@...hat.com, snitzer@...nel.org,
        dm-devel@...hat.com, song@...nel.org, dan.j.williams@...el.com,
        vishal.l.verma@...el.com, dave.jiang@...el.com,
        ira.weiny@...el.com, junxiao.bi@...cle.com,
        martin.petersen@...cle.com, kch@...dia.com,
        drbd-dev@...ts.linbit.com, linux-kernel@...r.kernel.org,
        linux-bcache@...r.kernel.org, linux-raid@...r.kernel.org,
        nvdimm@...ts.linux.dev, konrad.wilk@...cle.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [RFC] block: Change the granularity of io ticks from ms to ns

Hi,

在 2022/12/07 11:15, Ming Lei 写道:
> On Wed, Dec 07, 2022 at 10:19:08AM +0800, Yu Kuai wrote:
>> Hi,
>>
>> 在 2022/12/07 2:15, Gulam Mohamed 写道:
>>> Use ktime to change the granularity of IO accounting in block layer from
>>> milli-seconds to nano-seconds to get the proper latency values for the
>>> devices whose latency is in micro-seconds. After changing the granularity
>>> to nano-seconds the iostat command, which was showing incorrect values for
>>> %util, is now showing correct values.
>>
>> This patch didn't correct the counting of io_ticks, just make the
>> error accounting from jiffies(ms) to ns. The problem that util can be
>> smaller or larger still exist.
> 
> Agree.
> 
>>
>> However, I think this change make sense consider that error margin is
>> much smaller, and performance overhead should be minimum.
>>
>> Hi, Ming, how do you think?
> 
> I remembered that ktime_get() has non-negligible overhead, is there any
> test data(iops/cpu utilization) when running fio or t/io_uring on
> null_blk with this patch?

Yes, testing with null_blk is necessary, we don't want any performance
regression.

BTW, I thought it's fine because it's already used for tracking io
latency.
> 
> 
> Thanks,
> Ming
> 
> 
> .
>