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
