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Date: Wed, 17 Nov 2010 15:38:27 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wu Fengguang <fengguang.wu@intel.com>, Jan Kara <jack@suse.cz>,
        Li Shaohua <shaohua.li@intel.com>, Christoph Hellwig <hch@lst.de>,
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        linux-mm <linux-mm@kvack.org>, linux-fsdevel@vger.kernel.org,
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Subject: Re: [PATCH 06/13] writeback: bdi write bandwidth estimation
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On Thu, 18 Nov 2010 00:24:59 +0100
Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Wed, 2010-11-17 at 15:08 -0800, Andrew Morton wrote:
> > On Wed, 17 Nov 2010 12:27:26 +0800
> > Wu Fengguang <fengguang.wu@intel.com> wrote:
> > 
> > > +     w = min(elapsed / (HZ/100), 128UL);
> > 
> > I did try setting HZ=10 many years ago, and the kernel blew up.
> > 
> > I do recall hearing of people who set HZ very low, perhaps because
> > their huge machines were seeing performance prolems when the timer tick
> > went off.  Probably there's no need to do that any more.
> > 
> > But still, we shouldn't hard-wire the (HZ >= 100) assumption if we
> > don't absolutely need to, and I don't think it is absolutely needed
> > here.  
> 
> People who do cpu bring-up on very slow FPGAs also lower HZ as far as
> possible.

	grep -r "[^a-zA-Z0-9_]HZ[ ]*/[ ]*100[^0-9]" .

:-(