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

[Jens Axboe]

On 05/04/2015 02:33 PM, Shaohua Li wrote:
> 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.

That's fine, I'll grab 1-3 as-is. Thanks!

--
Jens Axboe

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