linux-kernel - Re: [PATCH 16/35] writeback: increase min pause time on concurrent dirtiers

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Date:	Tue, 14 Dec 2010 15:13:57 -0500
From:	Valdis.Kletnieks@...edu
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@....de>,
	Trond Myklebust <Trond.Myklebust@...app.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>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/35] writeback: increase min pause time on concurrent dirtiers

On Tue, 14 Dec 2010 19:55:08 +0100, Peter Zijlstra said:

> 10*HZ = 10 seconds
> (10*HZ) / 1024 ~= 10 milliseconds

from include/asm-generic/param.h (which is included by x86)

#ifdef __KERNEL__
# define HZ             CONFIG_HZ       /* Internal kernel timer frequency */
# define USER_HZ        100             /* some user interfaces are */
# define CLOCKS_PER_SEC (USER_HZ)       /* in "ticks" like times() */
#endif

Note that HZ isn't USER_HZ or CLOCKS_PER_SEC  - it's CONFIG_HZ, which last
I checked is still user-settable.  If not, then there needs to be a massive cleanup
of Kconfig and defconfig:

% grep HZ .config
CONFIG_NO_HZ=y
# CONFIG_HZ_100 is not set
# CONFIG_HZ_250 is not set
# CONFIG_HZ_300 is not set
CONFIG_HZ_1000=y
CONFIG_HZ=1000

So you're not guaranteed that 10*HZ is 10 seconds.  10*USER_HZ, sure. But not HZ.





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