Re: [patch,rfc v2] ext3/4: enhance fsync performance when using cfq

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Apr 08, 2010 at 01:00:45PM +0200, Jens Axboe wrote:
> On Wed, Apr 07 2010, Jeff Moyer wrote:
> > Hi again,
> > 
> > So, here's another stab at fixing this.  This patch is very much an RFC,
> > so do not pull it into anything bound for Linus.  ;-)  For those new to
> > this topic, here is the original posting:  http://lkml.org/lkml/2010/4/1/344
> > 
> > The basic problem is that, when running iozone on smallish files (up to
> > 8MB in size) and including fsync in the timings, deadline outperforms
> > CFQ by a factor of about 5 for 64KB files, and by about 10% for 8MB
> > files.  From examining the blktrace data, it appears that iozone will
> > issue an fsync() call, and will have to wait until it's CFQ timeslice
> > has expired before the journal thread can run to actually commit data to
> > disk.
> > 
> > The approach below puts an explicit call into the filesystem-specific
> > fsync code to yield the disk so that the jbd[2] process has a chance to
> > issue I/O.  This bring performance of CFQ in line with deadline.
> > 
> > There is one outstanding issue with the patch that Vivek pointed out.
> > Basically, this could starve out the sync-noidle workload if there is a
> > lot of fsync-ing going on.  I'll address that in a follow-on patch.  For
> > now, I wanted to get the idea out there for others to comment on.
> > 
> > Thanks a ton to Vivek for spotting the problem with the initial
> > approach, and for his continued review.
> 
> I like the concept, it's definitely useful (and your results amply
> demonstrate that). I was thinking if there was a way in through the ioc
> itself, rather than bdi -> queue and like you are doing. But I can't
> think of a nice way to do it, so this is probably as good as it gets.
> 

I think, one issue with ioc based approach will be that it will then call
yield operation on all the devices in the system where this context has ever
done any IO. With bdi based approach this call will remain limited to
a smaller set of devices.

Thanks
Vivek