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

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Message-ID: <33099.10057.qm@web32603.mail.mud.yahoo.com>
Date:	Wed, 17 Sep 2008 01:11:17 -0700 (PDT)
From:	Martin Knoblauch <knobi@...bisoft.de>
To:	Chris Snook <csnook@...hat.com>
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

----- Original Message ----

> 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>
> Sent: Tuesday, September 16, 2008 9:07:29 PM
> 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.
> 

 Now, I am pretty much aware that apparently things are not "ideal". Therefore my question. Because before doing patches, one has to analyze.

Cheers
Martin

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