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

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Message-Id: <20101117150837.a18d56c1.akpm@linux-foundation.org>
Date:	Wed, 17 Nov 2010 15:08:37 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	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>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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, 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.  
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