From: Jens Axboe <axboe@kernel.dk>
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>
Subject: Re: [RFC] block: Change the granularity of io ticks from ms to ns
Date: Wed, 7 Dec 2022 10:22:09 -0700 [thread overview]
Message-ID: <b8deb6fa-8a09-c1af-278f-24e66afe367d@kernel.dk> (raw)
In-Reply-To: <aadfc6d2-ad04-279c-a1d6-7f634d0b2c99@huaweicloud.com>
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
next prev parent reply other threads:[~2022-12-07 17:22 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-06 18:15 [RFC] block: Change the granularity of io ticks from ms to ns Gulam Mohamed
2022-12-06 19:31 ` Paul Menzel
2022-12-07 2:19 ` Yu Kuai
2022-12-07 3:15 ` Ming Lei
2022-12-07 13:09 ` Yu Kuai
2022-12-07 17:22 ` Jens Axboe [this message]
2022-12-07 17:08 ` Jens Axboe
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=b8deb6fa-8a09-c1af-278f-24e66afe367d@kernel.dk \
--to=axboe@kernel.dk \
--cc=agk@redhat.com \
--cc=christoph.boehmwalder@linbit.com \
--cc=colyli@suse.de \
--cc=dan.j.williams@intel.com \
--cc=dave.jiang@intel.com \
--cc=dm-devel@redhat.com \
--cc=drbd-dev@lists.linbit.com \
--cc=gulam.mohamed@oracle.com \
--cc=ira.weiny@intel.com \
--cc=junxiao.bi@oracle.com \
--cc=kch@nvidia.com \
--cc=kent.overstreet@gmail.com \
--cc=konrad.wilk@oracle.com \
--cc=lars.ellenberg@linbit.com \
--cc=linux-bcache@vger.kernel.org \
--cc=linux-block@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-raid@vger.kernel.org \
--cc=martin.petersen@oracle.com \
--cc=minchan@kernel.org \
--cc=ming.lei@redhat.com \
--cc=ngupta@vflare.org \
--cc=nvdimm@lists.linux.dev \
--cc=philipp.reisner@linbit.com \
--cc=senozhatsky@chromium.org \
--cc=snitzer@kernel.org \
--cc=song@kernel.org \
--cc=vishal.l.verma@intel.com \
--cc=yukuai1@huaweicloud.com \
--cc=yukuai3@huawei.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).