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@xxxxxxx/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@xxxxxxxx> SUSE Labs, CR
