From mboxrd@z Thu Jan  1 00:00:00 1970
From: Valdis.Kletnieks@vt.edu
Subject: Re: [PATCH 16/35] writeback: increase min pause time on concurrent dirtiers
Date: Tue, 14 Dec 2010 15:13:57 -0500
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References: <20101213144646.341970461@intel.com> <20101213150328.284979629@intel.com> <15881.1292264611@localhost> <20101214065133.GA6940@localhost> <14658.1292352152@localhost>
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Cc: Wu Fengguang <fengguang.wu@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jan Kara <jack@suse.cz>, Dave Chinner <david@fromorbit.com>,
	Christoph Hellwig <hch@lst.de>,
	Trond Myklebust <Trond.Myklebust@netapp.com>,
	"Theodore Ts'o" <tytso@mit.edu>,
	Chris Mason <chris.mason@oracle.com>,
	Mel Gorman <mel@csn.ul.ie>, Rik van Riel <riel@redhat.com>,
	KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
	Greg Thelen <gthelen@google.com>,
	Minchan Kim <minchan.kim@gmail.com>,
	linux-mm <linux-mm@kvack.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: Your message of "Tue, 14 Dec 2010 19:55:08 +0100."
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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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