linux-kernel - Re: [RFC for-6.2/block V2] block: Change the granularity of io ticks from ms to ns

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Message-ID: <09be5cbe-9251-d28c-e91a-3f2e5e9e99f2@nvidia.com>
Date:   Wed, 7 Dec 2022 23:17:12 +0000
From:   Chaitanya Kulkarni <chaitanyak@...dia.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Gulam Mohamed <gulam.mohamed@...cle.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
CC:     "philipp.reisner@...bit.com" <philipp.reisner@...bit.com>,
        "lars.ellenberg@...bit.com" <lars.ellenberg@...bit.com>,
        "christoph.boehmwalder@...bit.com" <christoph.boehmwalder@...bit.com>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "ngupta@...are.org" <ngupta@...are.org>,
        "senozhatsky@...omium.org" <senozhatsky@...omium.org>,
        "colyli@...e.de" <colyli@...e.de>,
        "kent.overstreet@...il.com" <kent.overstreet@...il.com>,
        "agk@...hat.com" <agk@...hat.com>,
        "snitzer@...nel.org" <snitzer@...nel.org>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "song@...nel.org" <song@...nel.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "junxiao.bi@...cle.com" <junxiao.bi@...cle.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        Chaitanya Kulkarni <chaitanyak@...dia.com>,
        "drbd-dev@...ts.linbit.com" <drbd-dev@...ts.linbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-bcache@...r.kernel.org" <linux-bcache@...r.kernel.org>,
        "linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "joe.jin@...cle.com" <joe.jin@...cle.com>
Subject: Re: [RFC for-6.2/block V2] block: Change the granularity of io ticks
 from ms to ns

On 12/7/22 15:08, Jens Axboe wrote:
> On 12/7/22 3:32?PM, Gulam Mohamed wrote:
>> As per the review comment from Jens Axboe, I am re-sending this patch
>> against "for-6.2/block".
>>
>>
>> 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:
> 
> My default peak testing runs at 122M IOPS. That's also the peak IOPS of
> the devices combined, and with iostats disabled. If I enabled iostats,
> then the performance drops to 112M IOPS. It's no longer device limited,
> that's a drop of about 8.2%.
> 

Wow, clearly not acceptable that's exactly I asked for perf
numbers :).

-ck