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
[off-list ref] wrote:

quoted

Hi ming

On 05/30/2018 05:13 PM, Ming Lei wrote:

quoted

quoted

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.

quoted

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