linux-kernel - Re: How to find out, what "pdflush" is working on

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Message-ID: <48D003F1.50101@redhat.com>
Date:	Tue, 16 Sep 2008 15:07:29 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Martin Knoblauch <knobi@...bisoft.de>
CC:	linux-kernel@...r.kernel.org,
	Peter zijlstra <a.p.zijlstra@...llo.nl>,
	Fengguang Wu <wfg@...l.ustc.edu.cn>
Subject: Re: How to find out, what "pdflush" is working on

Martin Knoblauch wrote:
> Hi,
> 
> I find the following comment in mm/pdflush.c
> 
> /* * The pdflush threads are worker threads for writing back dirty data. *
> Ideally, we'd like one thread per active disk spindle.  But the disk *
> topology is very hard to divine at this level.   Instead, we take * care in
> various places to prevent more than one pdflush thread from * performing
> writeback against a single filesystem.  pdflush threads * have the PF_FLUSHER
> flag set in current->flags to aid in this. */
> 
> Is there a way to find out what a certain instance of "pdflush" is working
> on? Like which block-device or which fliesystem it is writing to? I am still
> (2.6.27) trying to track down why writing a single file can make linux very
> sluggish and unresponsive. If that happens I usually see all possible 8
> "pdflush" threads being in "D"-state. According to above comment only one of
> them should be really busy.

The key word is "ideally".  We'd like it to work that way, but it doesn't. 
Patches to fix this are welcome.

-- Chris
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