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

[Ming Lei]

On Sat, May 09, 2020 at 04:57:48PM +0800, Baolin Wang wrote:
> On Sat, May 9, 2020 at 7:22 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> >
> > Hi Sagi,
> >
> > On Fri, May 08, 2020 at 03:19:45PM -0700, Sagi Grimberg wrote:
> > > Hey Ming,
> > >
> > > > > Would it make sense to elevate this flag to a request_queue flag
> > > > > (QUEUE_FLAG_ALWAYS_COMMIT)?
> > > >
> > > > request queue flag usually is writable, however this case just needs
> > > > one read-only flag, so I think it may be better to make it as
> > > > tagset/hctx flag.
> > >
> > > I actually intended it to be writable.
> > >
> > > > > I'm thinking of a possibility that an I/O scheduler may be used
> > > > > to activate this functionality rather than having the driver set
> > > > > it necessarily...
> > > >
> > > > Could you explain a bit why I/O scheduler should activate this
> > > > functionality?
> > >
> > > Sure, I've recently seen some academic work showing the benefits
> > > of batching in tcp/ip based block drivers. The problem with the
> > > approaches taken is that I/O scheduling is exercised deep down in the
> > > driver, which is not the direction I'd like to go if we are want
> > > to adopt some of the batching concepts.
> > >
> > > I spent some (limited) time thinking about this, and it seems to
> > > me that there is an opportunity to implement this as a dedicated
> > > I/O scheduler, and tie it to driver specific LLD stack optimizations
> > > (net-stack for example) relying on the commit_rq/bd->last hints.
> > >
> > > When scanning the scheduler code, I noticed exactly the phenomenon that
> > > this patchset is attempting to solve and Christoph referred me to it.
> > > Now I'm thinking if we can extend this batching optimization for both
> > > use-cases.
> >
> > Got it, thanks for the sharing.
> >
> > >
> > > > batching submission may be good for some drivers, and currently
> > > > we only do it in limited way. One reason is that there is extra
> > > > cost for full batching submission, such as this patch requires
> > > > one extra .commit_rqs() for each dispatch, and lock is often needed
> > > > in this callback.
> > >
> > > That is not necessarily the case at all.
> >
> > So far, all in-tree .commit_rqs() implementation requires lock.
> >
> > >
> > > > IMO it can be a win for some slow driver or device, but may cause
> > > > a little performance drop for fast driver/device especially in workload
> > > > of not-batching submission.
> > >
> > > 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.

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.


thanks, 
Ming