Re: [PATCH 4/5] blk-mq: do limited block plug for multiple queue case

[Shaohua Li]

On Mon, May 04, 2015 at 01:46:49PM -0600, Jens Axboe wrote:
> On 05/04/2015 01:40 PM, Shaohua Li wrote:
> >On Fri, May 01, 2015 at 04:16:04PM -0400, Jeff Moyer wrote:
> >>Shaohua Li <shli@xxxxxx> writes:
> >>
> >>>plug is still helpful for workload with IO merge, but it can be harmful
> >>>otherwise especially with multiple hardware queues, as there is
> >>>(supposed) no lock contention in this case and plug can introduce
> >>>latency. For multiple queues, we do limited plug, eg plug only if there
> >>>is request merge. If a request doesn't have merge with following
> >>>request, the requet will be dispatched immediately.
> >>>
> >>>This also fixes a bug. If we directly issue a request and it fails, we
> >>>use blk_mq_merge_queue_io(). But we already assigned bio to a request in
> >>>blk_mq_bio_to_request. blk_mq_merge_queue_io shouldn't run
> >>>blk_mq_bio_to_request again.
> >>
> >>Good catch.  Might've been better to split that out first for easy
> >>backport to stable kernels, but I won't hold you to that.
> >
> >It's not a severe bug, but I don't mind. Jens, please let me know if I
> >should split the patch into 2 patches.
> 
> I don't care that much for this particular case. But since one/more
> of the others need respin anyway, might be prudent to split it up in
> any case.

ok, done. I'll repost the patch 4/5. Please let me know if I should
repost 1-3.