Re: background on the ext3 batching performance issue

[Jan Kara <jack@suse.cz>]

> On Thursday 28 February 2008, Ric Wheeler wrote:
> 
> [ fsync batching can be slow ]
> 
> > One more thought - what we really want here is to have a sense of the
> > latency of the device. In the S-ATA disk case, this optimization works
> > well for batching since we "spend" an extra 4ms worst case in the chance
> > of combining multiple, slow 18ms operations.
> >
> > With the clariion box we tested, the optimization fails badly since the
> > cost is only 1.3 ms so we optimize by waiting 3-4 times longer than it
> > would take to do the operation immediately.
> >
> > This problem has also seemed to me to be the same problem that IO
> > schedulers do with plugging - we want to dynamically figure out when to
> > plug and unplug here without hard coding in device specific tunings.
> >
> > If we bypass the snippet for multi-threaded writers, we would probably
> > slow down this workload on normal S-ATA/ATA drives (or even higher
> > performance non-RAID disks).
> 
> It probably makes sense to keep track of the average number of writers we are 
> able to gather into a transcation.  There are lots of similar workloads where 
> we have a pool of procs doing fsyncs and the size of the transaction or the 
> number of times we joined a running transaction will be fairly constant.
  I'm probably missing something, but what are you trying to say? Either we
wait for writers and the number of writes is higher, or we don't wait and the
number of writes in a transaction is lower...

								Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs