linux-kernel - Re: [PATCH 06/13] writeback: bdi write bandwidth estimation

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Date:	Thu, 18 Nov 2010 00:24:59 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Li Shaohua <shaohua.li@...el.com>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/13] writeback: bdi write bandwidth estimation

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@...el.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.
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