linux-kernel - Re: [RFC PATCH] mm: let the bdi_writeout fraction respond more quickly

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Date:	Thu, 17 Jun 2010 12:39:56 +0100
From:	Richard Kennedy <richard@....demon.co.uk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC PATCH] mm: let the bdi_writeout fraction respond more
 quickly

On Wed, 2010-06-16 at 20:54 +0200, Peter Zijlstra wrote:
> On Mon, 2010-06-14 at 15:44 +0100, Richard Kennedy wrote:
> > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > index 2fdda90..315dd04 100644
> > > --- a/mm/page-writeback.c
> > > +++ b/mm/page-writeback.c
> > > @@ -144,7 +144,7 @@ static int calc_period_shift(void)
> > >       else
> > >               dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
> > >                               100;
> > > -     return 2 + ilog2(dirty_total - 1);
> > > +     return ilog2(dirty_total - 1) - 4;
> > >  } 
> 
> IIRC I suggested similar things in the past and all we needed to do was
> find people doing the measurements on different bits of hardware or so..
> 
> I don't have any problems with the approach, all we need to make sure is
> that we never return 0 or a negative number (possibly ensure a minimum
> positive shift value).

Yep that sounds reasonable. would minimum shift of 4 be ok ?

something like

	max ( (ilog2(dirty_total - 1)- 4) , 4);

Unfortunately volunteers don't seem to be leaping out of the woodwork,
maybe Andrew could be persuaded to try this in his tree for a while and
see if any one squeaks ?

regards
Richard

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