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Message-ID: <eaf4f9a8-dfc6-402e-4a1a-732034d1512d@kernel.dk>
Date:   Wed, 7 Dec 2022 10:08:50 -0700
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Subject: Re: [RFC] block: Change the granularity of io ticks from ms to ns
To:     Gulam Mohamed <gulam.mohamed@oracle.com>,
        linux-block@vger.kernel.org
Cc:     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
References: <20221206181536.13333-1-gulam.mohamed@oracle.com>
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From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <20221206181536.13333-1-gulam.mohamed@oracle.com>
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On 12/6/22 11:15?AM, Gulam Mohamed wrote:
> 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.
> 
> We did not work on the patch to drop the logic for
> STAT_PRECISE_TIMESTAMPS yet. Will do it if this patch is ok.
> 
> The iostat command was run after starting the fio with following command
> on an NVME disk. For the same fio command, the iostat %util was showing
> ~100% for the disks whose latencies are in the range of microseconds.
> With the kernel changes (granularity to nano-seconds), the %util was
> showing correct values. Following are the details of the test and their
> output:

As mentioned, this will most likely have a substantial performance
impact. I'd test it, but your patch is nowhere near applying to the
current block tree. Please resend it against for-6.2/block so it can
get tested.

-- 
Jens Axboe