Re: [RFC PATCH v2 1/7] block: Extand commit_rqs() to do batch processing

[Sagi Grimberg]

> > > > You're mostly correct. This is exactly why an I/O scheduler may be
> > > > applicable here IMO. Mostly because I/O schedulers tend to optimize for
> > > > something specific and always present tradeoffs. Users need to
> > > > understand what they are optimizing for.
> > > >
> > > > Hence I'd say this functionality can definitely be available to an I/O
> > > > scheduler should one exist.
> > >
> > > I guess it is just that there can be multiple requests available from
> > > scheduler queue. Actually it can be so for other non-nvme drivers in
> > > case of none, such as SCSI.
> > >
> > > Another way is to use one per-task list(such as plug list) to hold the
> > > requests for dispatch, then every drivers may see real .last flag, so they
> > > may get chance for optimizing batch queuing. I will think about the
> > > idea further and see if it is really doable.
> >
> > How about my RFC v1 patch set[1], which allows dispatching more than
> > one request from the scheduler to support batch requests?
> >
> > [1]
> > https://lore.kernel.org/patchwork/patch/1210034/
> > https://lore.kernel.org/patchwork/patch/1210035/
>
> Basically, my idea is to dequeue request one by one, and for each
> dequeued request:
>
> - we try to get a budget and driver tag, if both succeed, add the
> request to one per-task list which can be stored in stack variable,
> then continue to dequeue more request
>
> - if either budget or driver tag can't be allocated for this request,
> marks the last request in the per-task list as .last, and send the
> batching requests stored in the list to LLD
>
> - when queueing batching requests to LLD, if one request isn't queued
> to driver successfully, calling .commit_rqs() like before, meantime
> adding the remained requests in the per-task list back to scheduler
> queue or hctx->dispatch.

Sounds good to me.

> One issue is that this way might degrade sequential IO performance if
> the LLD just tells queue busy to blk-mq via return value of .queue_rq(),
> so I guess we still may need one flag, such as BLK_MQ_F_BATCHING_SUBMISSION.

Why is that degrading sequential I/O performance? because the specific
device will do better without batching submissions? If so, the driver
is not obligated to respect the bd->last/.commit_rqs, so if that is the
case, it should just ignore it.