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

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Date:	Fri, 15 May 2009 11:35:55 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"Peter W. Morreale" <pmorreale@...ell.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, May 13 2009, Peter W. Morreale wrote:
> 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.  

OK. In general I think it's a pretty bad idea to add such knobs before
there are specific use cases, as we have to maintain them forever. I
didn't track where this patch came from, I just spotted it in mainline
during the merge window.

> 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?

Sooner rather than later. I've been working on it the past few days, I
needed to make some fundemental changes to support WB_SYNC_ALL and
sync(1) properly, unfortunately. I'll be posting an updated patchset
early next week.

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

Good, I have reverted the commit in my for-linus branch and will be
asking Linus to pull that soonish.

-- 
Jens Axboe

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