From: Jens Axboe <axboe@kernel.dk>
To: Keith Busch <kbusch@kernel.org>,
Chaitanya Kulkarni <chaitanyak@nvidia.com>
Cc: Gulam Mohamed <gulam.mohamed@oracle.com>,
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Subject: Re: [RFC for-6.2/block V2] block: Change the granularity of io ticks from ms to ns
Date: Wed, 7 Dec 2022 19:55:30 -0700 [thread overview]
Message-ID: <4d118f20-9006-0af9-8d97-0d28d85a3585@kernel.dk> (raw)
In-Reply-To: <Y5Exa1TV/2VLcEWR@kbusch-mbp>
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
next prev parent reply other threads:[~2022-12-08 2:55 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-07 22:32 [RFC for-6.2/block V2] block: Change the granularity of io ticks from ms to ns Gulam Mohamed
2022-12-07 23:02 ` Chaitanya Kulkarni
2022-12-07 23:08 ` Jens Axboe
2022-12-07 23:17 ` Chaitanya Kulkarni
2022-12-08 0:35 ` Keith Busch
2022-12-08 2:55 ` Jens Axboe [this message]
2022-12-08 0:36 ` Ming Lei
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