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
