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Subject: Re: [PATCH 06/13] writeback: bdi write bandwidth estimation
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>, Jan Kara <jack@suse.cz>,
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Date: Thu, 18 Nov 2010 00:24:59 +0100
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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.