Hi Ming Thanks for your kindly and detailed response. :) On 05/30/2018 05:44 PM, Ming Lei wrote: > On Wed, May 30, 2018 at 5:20 PM, jianchao.wang > <jianchao.w.wang@xxxxxxxxxx> wrote: >> Hi ming >> >> On 05/30/2018 05:13 PM, Ming Lei wrote: >>>> Yes, it maybe good for merging of 'none', because the rq_list is split into 3 >>>> lists, and not need to iterate the whole rq_list any more. >>>> But what's about the dispatch when there is no io scheduler. >>> blk_mq_flush_busy_ctxs() and blk_mq_dequeue_from_ctx() should work >>> fine in case of 'none' if per-domain list is added to ctx. Then we can make >>> none to be a bit fair on READ/WRITE. >>> >> >> But how to determine when to dispatch READ, WRITE or other more, when there is no io scheduler ? >> > > For blk-mq, no io scheduler means 'none' actually, and it works like a > scheduler too, but just shares driver tags, IMO. > > Wrt. the current code of 'none', blk-mq just picks up one request from > ctx->rq_list > directly in FIFO style. If READ/WRITE lists are introduced, only > blk_mq_dequeue_from_ctx() is effected, there are several choices > left for us: > > 1) keep the FIFO style of current behaviour by using req->start_time_ns > > 2) READ/WRIRE fair style by picking up request from the lists in round-robin > order > > 3) or others > > It just will make more choices for us, :-) OK, I got the point. But is it necessary to introduce kind of dispatch policy which is more complicated than current simple FIFO style in ctx rq_list dispatching ? If we have this kind of requirement, why not introduce an io scheduler ? ITOW, shouldn't we keep the blk-mq core code as simple as possible, and put most of the policy into io scheduler ? Thanks Jianchao
