From: Jens Axboe <axboe@kernel.dk>
To: Jan Kara <jack@suse.cz>
Cc: linux-block@vger.kernel.org, Ming Lei <ming.lei@redhat.com>,
Paolo Valente <paolo.valente@linaro.org>
Subject: Re: [PATCH v2] block: Do not pull requests from the scheduler when we cannot dispatch them
Date: Wed, 16 Jun 2021 09:43:06 -0600 [thread overview]
Message-ID: <d61be138-7fc2-76d6-d01f-6ec555e46f29@kernel.dk> (raw)
In-Reply-To: <20210616154038.GA18520@quack2.suse.cz>
On 6/16/21 9:40 AM, Jan Kara wrote:
> On Thu 03-06-21 12:47:21, Jan Kara wrote:
>> Provided the device driver does not implement dispatch budget accounting
>> (which only SCSI does) the loop in __blk_mq_do_dispatch_sched() pulls
>> requests from the IO scheduler as long as it is willing to give out any.
>> That defeats scheduling heuristics inside the scheduler by creating
>> false impression that the device can take more IO when it in fact
>> cannot.
>>
>> For example with BFQ IO scheduler on top of virtio-blk device setting
>> blkio cgroup weight has barely any impact on observed throughput of
>> async IO because __blk_mq_do_dispatch_sched() always sucks out all the
>> IO queued in BFQ. BFQ first submits IO from higher weight cgroups but
>> when that is all dispatched, it will give out IO of lower weight cgroups
>> as well. And then we have to wait for all this IO to be dispatched to
>> the disk (which means lot of it actually has to complete) before the
>> IO scheduler is queried again for dispatching more requests. This
>> completely destroys any service differentiation.
>>
>> So grab request tag for a request pulled out of the IO scheduler already
>> in __blk_mq_do_dispatch_sched() and do not pull any more requests if we
>> cannot get it because we are unlikely to be able to dispatch it. That
>> way only single request is going to wait in the dispatch list for some
>> tag to free.
>>
>> Reviewed-by: Ming Lei <ming.lei@redhat.com>
>> Signed-off-by: Jan Kara <jack@suse.cz>
>> ---
>> block/blk-mq-sched.c | 12 +++++++++++-
>> block/blk-mq.c | 2 +-
>> block/blk-mq.h | 2 ++
>> 3 files changed, 14 insertions(+), 2 deletions(-)
>>
>> Jens, can you please merge the patch? Thanks!
>
> Ping Jens?
Didn't I reply? It's merged for a while:
https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.14/block&id=613471549f366cdf4170b81ce0f99f3867ec4d16
--
Jens Axboe
next prev parent reply other threads:[~2021-06-16 15:48 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-03 10:47 [PATCH v2] block: Do not pull requests from the scheduler when we cannot dispatch them Jan Kara
2021-06-03 18:01 ` Jens Axboe
2021-06-07 10:05 ` Hannes Reinecke
2021-06-07 11:41 ` Jan Kara
2021-06-16 15:40 ` Jan Kara
2021-06-16 15:43 ` Jens Axboe [this message]
2021-06-16 15:51 ` Jan Kara
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=d61be138-7fc2-76d6-d01f-6ec555e46f29@kernel.dk \
--to=axboe@kernel.dk \
--cc=jack@suse.cz \
--cc=linux-block@vger.kernel.org \
--cc=ming.lei@redhat.com \
--cc=paolo.valente@linaro.org \
/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 NNTP newsgroup(s).