Re: [PATCH 0/11] Per-bdi writeback flusher threads v9

[Frederic Weisbecker]

On Sat, Jun 06, 2009 at 02:23:40AM +0200, Jan Kara wrote:
> On Fri 05-06-09 20:18:15, Chris Mason wrote:
> > On Fri, Jun 05, 2009 at 11:14:38PM +0200, Jan Kara wrote:
> > > On Fri 05-06-09 21:15:28, Jens Axboe wrote:
> > > > On Fri, Jun 05 2009, Frederic Weisbecker wrote:
> > > > > The result with noop is even more impressive.
> > > > > 
> > > > > See: http://kernel.org/pub/linux/kernel/people/frederic/dbench-noop.pdf
> > > > > 
> > > > > Also a comparison, noop with pdflush against noop with bdi writeback:
> > > > > 
> > > > > http://kernel.org/pub/linux/kernel/people/frederic/dbench-noop-cmp.pdf
> > > > 
> > > > OK, so things aren't exactly peachy here to begin with. It may not
> > > > actually BE an issue, or at least now a new one, but that doesn't mean
> > > > that we should not attempt to quantify the impact.
> > >   What looks interesting is also the overall throughput. With pdflush we
> > > get to 2.5 MB/s + 26 MB/s while with per-bdi we get to 2.7 MB/s + 13 MB/s.
> > > So per-bdi seems to be *more* fair but throughput suffers a lot (which
> > > might be inevitable due to incurred seeks).
> > >   Frederic, how much does dbench achieve for you just on one partition
> > > (test both consecutively if possible) with as many threads as have those
> > > two dbench instances together? Thanks.
> > 
> > Is the graph showing us dbench tput or disk tput?  I'm assuming it is
> > disk tput, so bdi may just be writing less?
>   Good, question. I was assuming dbench throughput :).
> 
> 									Honza


Yeah it's dbench. May be that's not the right tool to measure the writeback
layer, even though dbench results are necessarily influenced by the writeback
behaviour.

May be I should use something else?

Note that if you want I can put some surgicals trace_printk()
in fs/fs-writeback.c


> 
> -- 
> Jan Kara <jack@xxxxxxx>
> SUSE Labs, CR

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