Re: [patch 1/4] raid1: directly dispatch write request if no bitmap

[NeilBrown <neilb@xxxxxxx>]

On Thu, 24 May 2012 10:54:25 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:

> On Thu, May 24, 2012 at 12:21:12PM +1000, NeilBrown wrote:
> > On Wed, 23 May 2012 15:26:20 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:
> > 
> > > In raid1, all write requests are dispatched in raid1d thread. In fast storage,
> > > the raid1d thread is a bottleneck, because it dispatches request too slow. Also
> > > raid1d thread migrates freely, which makes request completion cpu not match
> > > with submission cpu even driver/block layer has such capability. This will
> > > cause bad cache issue.
> > > 
> > > If no bitmap, there is no point to queue bio to a thread and dispatch it in the
> > > thread. Directly dispatching bio doesn't impact correctness and removes above
> > > bottleneck.
> > > 
> > > Multiple threads dispatch requests could potentially reduce request merge and
> > > increase lock contention. For slow stroage, we just worry about request merge.
> > > Caller of .make_request should already have correct block plug set, which will
> > > take care of request merge and locking just like accessing raw device, so we
> > > don't need worry about this too much.
> > > 
> > > In a 4k randwrite test with a 2 disks setup, below patch can provide 20% ~ 50%
> > > performance improvements depending on numa binding.
> > > 
> > > Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
> > > ---
> > >  drivers/md/raid1.c |   11 +++++++----
> > >  1 file changed, 7 insertions(+), 4 deletions(-)
> > > 
> > > Index: linux/drivers/md/raid1.c
> > > ===================================================================
> > > --- linux.orig/drivers/md/raid1.c	2012-05-22 13:50:26.989820654 +0800
> > > +++ linux/drivers/md/raid1.c	2012-05-22 13:56:46.117054559 +0800
> > > @@ -1187,10 +1187,13 @@ read_again:
> > >  		mbio->bi_private = r1_bio;
> > >  
> > >  		atomic_inc(&r1_bio->remaining);
> > > -		spin_lock_irqsave(&conf->device_lock, flags);
> > > -		bio_list_add(&conf->pending_bio_list, mbio);
> > > -		conf->pending_count++;
> > > -		spin_unlock_irqrestore(&conf->device_lock, flags);
> > > +		if (bitmap) {
> > > +			spin_lock_irqsave(&conf->device_lock, flags);
> > > +			bio_list_add(&conf->pending_bio_list, mbio);
> > > +			conf->pending_count++;
> > > +			spin_unlock_irqrestore(&conf->device_lock, flags);
> > > +		} else
> > > +			generic_make_request(mbio);
> > >  	}
> > >  	/* Mustn't call r1_bio_write_done before this next test,
> > >  	 * as it could result in the bio being freed.
> > 
> > This looks like it should be 'obviously correct' but unfortunately it isn't.
> > 
> > If this raid1 is beneath a dm device (e.g. LVM), then generic_make_request
> > will queue the request internally and not actually start it.
> > In particular this means that the cloned bio has no chance of being released
> > before the next clone_bio call which is made on the next time around the loop.
> > 
> > This can lead to a deadlock as the clone_bio() might be waiting for that
> > first cloned bio to be released (if memory is really tight).
> > 
> > i.e. when allocating multiple bios from a mempool, we need to arrange for
> > them to be submitted by a separate thread.
> 
> If there isn't block plug, generic_make_request will dispatch the request
> directly.

Not necessarily.  It might queue it:
	if (current->bio_list) {
		bio_list_add(current->bio_list, bio);
		return;
	}

NeilBrown



 If there is, when clone_bio() waits for memory, it will
> sleep/schedule, which will trigger block unplug and dispatch the first bio, so
> no deadlock to me. Am I misunderstanding you?
> 
> Thanks,
> Shaohua

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