Re: [PATCH 1/2] block: Do not merge recursively in elv_attempt_insert_merge()

[Jan Kara <jack@suse.cz>]

On Fri 21-05-21 21:12:14, Ming Lei wrote:
> On Fri, May 21, 2021 at 01:53:54PM +0200, Jan Kara wrote:
> > On Fri 21-05-21 08:42:16, Ming Lei wrote:
> > > 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
> > 
> > Oh, request merging definitely does make a difference. But the elevator
> > hash & merge logic I'm modifying here is used only by BFQ and MQ-DEADLINE
> > AFAICT. And these IO schedulers will already call blk_mq_sched_try_merge()
> > from their \.bio_merge handler which gets called from blk_mq_submit_bio().
> > So all the merging that can happen in the code I remove should have already
> > happened. Or am I missing something?
> 
> There might be at least two reasons:
> 
> 1) when .bio_merge() is called, some requests are kept in plug list, so
> the bio may not be merged to requests in scheduler queue; when flushing plug
> list and inserts these requests to scheduler queue, we have to try to
> merge them further

Oh, right, I forgot that plug list stores already requests, not bios.

> 2) only blk_mq_sched_try_insert_merge() is capable of doing aggressive
> request merge, such as, when req A is merged to req B, the function will
> continue to try to merge req B with other in-queue requests, until no
> any further merge can't be done; neither blk_mq_sched_try_merge() nor
> blk_attempt_plug_merge can do such aggressive request merge.

Yes, fair point. I was thinking only about a few requests but it the request
sequence is like 0 2 4 6 ... 2n 1 3 5 7 ... 2n+1, then bio merging will
result in 'n' requests while request merging will be able to get it down to
1 request.

I'll keep the recursive merge and pass back list of requests to free
instead. Thanks for explanations!

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR