Re: BIO_RW_SYNCIO

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jul 28, 2010 at 08:42:06AM -0400, Mikulas Patocka wrote:
> > > I took the traces and placed them at 
> > > http://people.redhat.com/mpatocka/data/blktrace/
> > > 
> > > It shows that WRITE requests are merged without SYNCIO flags and are not 
> > > merged if SYNCIO is used.
> > 
> > Yes you are right. So I think following is happening.
> > 
> > Key is that requests don't get merged once they are on dispatch list. They
> > get merged only when they are still sitting in some cfq queue and are with
> > elevator.
> >
> > In case of sync IO, both reads and writes are on single cfq queue. We are
> > driving good dispatch list depth (drv=65). That means there are 65 reads
> > and writes on dispatch list and none of the new requests can be merged with
> > those.
> > 
> > In case of async IO, reads and writes are going on different cfq queues.
> > While reads are being dispatched from one queue, writes are sitting in 
> > CFQ and are open to merge. That's the reason we are seeing lot more WRITE
> > merging with async case.
> > 
> > Not sure what we can do about it though. But had a couple of questions.
> > 
> > - You seem to be issuing lots of 4K size adjacent READS and WRITES. Is
> >   there a way that you can club these together and issue a bigger request.
> 
> It is possible, but it would mean major code size increase (replicate the 
> merge functionality in dm-kcopyd). We don't have problems with CPU time 
> consumption, so we are not planning it now.
> 
> It just simpler to turn off BIO_RW_SYNCIO. I also turned off BIO_RW_UNPLUG 
> and unplug the queue after more requests. It improves performance to 
> 22MB/s.
> 

I am not very sure how effective unplug thing is because it works only
if there is no IO happening in device. From blktraces it looks that device
is continuously busy.

I guess that not marking writes as sync is probably best in this case.
Though it will be interesting how does this look in presence of other
buffered writers on some other partition on device. I am not sure how
common case that is.

Are you seeing same issue with deadline also? I guess deadline might
run into same issue and beacause there is no idling logic there, I 
think even turning off BIO_RW_SYNCIO is not going to help.

> > - What kind of device this is where request queue depth is 65. Can you
> >   try reducing request queue depth to say 16 and see if things improve
> >   a bit. (/sys/block/<dev>/device/queue_depth).
> 
> Seagate U320 SCSI disk on MPT controller. It has 64 tags.
> 
> When I reduced the number of tags, it improved performance, 16 was good 
> (19MB/s), reducing it to 4 or 1 improved it even more (22MB/s).

Ok. I looked at the code again and realized that cfq allows unlimited dispatch
from a queue if there are no other competing queues. That's the reason in this
case we are allowing 64 request dispatch from single queue at a time.

Vivek