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Date:   Wed, 7 Dec 2022 11:15:43 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     Gulam Mohamed <gulam.mohamed@oracle.com>,
        linux-block@vger.kernel.org, axboe@kernel.dk,
        philipp.reisner@linbit.com, lars.ellenberg@linbit.com,
        christoph.boehmwalder@linbit.com, minchan@kernel.org,
        ngupta@vflare.org, senozhatsky@chromium.org, colyli@suse.de,
        kent.overstreet@gmail.com, agk@redhat.com, snitzer@kernel.org,
        dm-devel@redhat.com, song@kernel.org, dan.j.williams@intel.com,
        vishal.l.verma@intel.com, dave.jiang@intel.com,
        ira.weiny@intel.com, junxiao.bi@oracle.com,
        martin.petersen@oracle.com, kch@nvidia.com,
        drbd-dev@lists.linbit.com, linux-kernel@vger.kernel.org,
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        "yukuai (C)" <yukuai3@huawei.com>
Subject: Re: [RFC] block: Change the granularity of io ticks from ms to ns
Message-ID: <Y5AFX4sxLRLV4uSt@T590>
References: <20221206181536.13333-1-gulam.mohamed@oracle.com>
 <936a498b-19fe-36d5-ff32-817f153e57e3@huaweicloud.com>
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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?


Thanks, 
Ming