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Message-ID: <b8deb6fa-8a09-c1af-278f-24e66afe367d@kernel.dk>
Date:   Wed, 7 Dec 2022 10:22:09 -0700
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Subject: Re: [RFC] block: Change the granularity of io ticks from ms to ns
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To:     Yu Kuai <yukuai1@huaweicloud.com>, Ming Lei <ming.lei@redhat.com>
Cc:     Gulam Mohamed <gulam.mohamed@oracle.com>,
        linux-block@vger.kernel.org, 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,
        linux-bcache@vger.kernel.org, linux-raid@vger.kernel.org,
        nvdimm@lists.linux.dev, konrad.wilk@oracle.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20221206181536.13333-1-gulam.mohamed@oracle.com>
 <936a498b-19fe-36d5-ff32-817f153e57e3@huaweicloud.com>
 <Y5AFX4sxLRLV4uSt@T590>
 <aadfc6d2-ad04-279c-a1d6-7f634d0b2c99@huaweicloud.com>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <aadfc6d2-ad04-279c-a1d6-7f634d0b2c99@huaweicloud.com>
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On 12/7/22 6:09 AM, Yu Kuai wrote:
> 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.

null_blk is fine as a substitute, but I'd much rather run this on my
test bench with actual IO and devices.

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

Reading a nsec timestamp is a LOT more expensive than reading jiffies,
which is essentially free. If you look at the amount of work that's
gone into minimizing ktime_get() for the fast path in the IO stack,
then that's a testament to that.

So that's a very bad assumption, and definitely wrong.

-- 
Jens Axboe