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

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Date:   Wed, 7 Dec 2022 19:55:30 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Keith Busch <kbusch@...nel.org>,
        Chaitanya Kulkarni <chaitanyak@...dia.com>
Cc:     Gulam Mohamed <gulam.mohamed@...cle.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "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>,
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        "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 5:35?PM, Keith Busch wrote:
> On Wed, Dec 07, 2022 at 11:17:12PM +0000, Chaitanya Kulkarni wrote:
>> On 12/7/22 15:08, Jens Axboe wrote:
>>>
>>> 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 :).
> 
> For the record, we did say per-io ktime_get() has a measurable
> performance harm and should be aggregated.
> 
>   https://www.spinics.net/lists/linux-block/msg89937.html

Yes, I iterated that in the v1 posting as well, and mentioned it was the
reason the time batching was done. From the results I posted, if you
look at a profile of the run, here are the time related additions:

+   27.22%  io_uring  [kernel.vmlinux]  [k] read_tsc
+    4.37%  io_uring  [kernel.vmlinux]  [k] ktime_get

which are #1 and $4, respectively. That's a LOT of added overhead. Not
sure why people think time keeping is free, particularly high
granularity time keeping. It's definitely not, and adding 2-3 per IO is
very noticeable.

-- 
Jens Axboe