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

[Ming Lei]

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.


Thanks,
Ming