linux-kernel - Re: do we really want to export more pdflush details in sysctls

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Date:	Wed, 13 May 2009 08:30:24 -0600
From:	"Peter W. Morreale" <pmorreale@...ell.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Christoph Hellwig <hch@....de>, torvalds@...l.org, akpm@...l.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: do we really want to export more pdflush details in sysctls

On Wed, 2009-05-13 at 15:08 +0200, Jens Axboe wrote:
> On Wed, May 13 2009, Christoph Hellwig wrote:
> > Hi all,
> > 
> > commit fafd688e4c0c34da0f3de909881117d374e4c7af titled
> > "mm: add /proc controls for pdflush threads" adds two more sysctl
> > variables exposing details about pdflush threads.  At the same time
> > Jens Axboe is working on the per-bdi writeback patchset which will
> > hopefull soon get rid of the pdflush threads in their current form.
> > 
> > Is it really a good idea to expose more details now or should we revert
> > this patch before 2.6.30 is out?
> 
> Pained me as well when updating the patchset. I see little value in
> these knobs as it is, I'm imagining that the submitter must have had a
> use case where it made some difference?
> 

No, I didn't.  The rational was as explained in the commit log, merely
that one size (eg: 2-8 threads) didn't fit all cases, so give the admin
a chance at tuning w/o having to recompile.  

More importantly, I didn't know that Jens was working on significant
changes to writeback.  This is sorely needed as from what I see in the
code, writeback is very unfair to 'fast' block devices (when both 'fast'
and 'slow' devices co-exist), and consequently, the apps that reference
them.  

Jens: When do you expect to complete the per-bdi patchset?

In any event, it is not a good idea to expose knobs that will soon be
obviated so please pull the patch. 

Thanks,
-PWM

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