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From: Ming Lei <ming.lei@redhat.com>
To: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Khazhy Kumykov <khazhy@google.com>,
	Paolo Valente <paolo.valente@linaro.org>
Subject: Re: [PATCH 1/2] block: Do not merge recursively in elv_attempt_insert_merge()
Date: Fri, 21 May 2021 08:42:16 +0800	[thread overview]
Message-ID: <YKcB6Hxhe3a1St31@T590> (raw)
In-Reply-To: <20210520223353.11561-2-jack@suse.cz>

On Fri, May 21, 2021 at 12:33:52AM +0200, Jan Kara wrote:
> Most of the merging happens at bio level. There should not be much
> merging happening at request level anymore. Furthermore if we backmerged
> a request to the previous one, the chances to be able to merge the
> result to even previous request are slim - that could succeed only if
> requests were inserted in 2 1 3 order. Merging more requests in

Right, but some workload has this kind of pattern.

For example of qemu IO emulation, it often can be thought as single job,
native aio, direct io with high queue depth. IOs is originated from one VM, but
may be from multiple jobs in the VM, so bio merge may not hit much because of IO
emulation timing(virtio-scsi/blk's MQ, or IO can be interleaved from multiple
jobs via the SQ transport), but request merge can really make a difference, see
recent patch in the following link:

https://lore.kernel.org/linux-block/3f61e939-d95a-1dd1-6870-e66795cfc1b1@suse.de/T/#t

> elv_attempt_insert_merge() will be difficult to handle when we want to
> pass requests to free back to the caller of
> blk_mq_sched_try_insert_merge(). So just remove the possibility of
> merging multiple requests in elv_attempt_insert_merge().

This way will cause regression.


Thanks, 
Ming


  reply	other threads:[~2021-05-21  0:42 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-20 22:33 [PATCH 0/2] block: Fix deadlock when merging requests with BFQ Jan Kara
2021-05-20 22:33 ` [PATCH 1/2] block: Do not merge recursively in elv_attempt_insert_merge() Jan Kara
2021-05-21  0:42   ` Ming Lei [this message]
2021-05-21 11:53     ` Jan Kara
2021-05-21 13:12       ` Ming Lei
2021-05-21 13:44         ` Jan Kara
2021-05-20 22:33 ` [PATCH 2/2] blk: Fix lock inversion between ioc lock and bfqd lock Jan Kara
2021-05-21  0:57   ` Ming Lei
2021-05-21  3:29     ` Khazhy Kumykov
2021-05-21  6:54       ` Ming Lei
2021-05-21 12:05         ` Jan Kara
2021-05-21 13:36           ` Ming Lei
2021-05-21 13:47             ` Jan Kara

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