linux-kernel - Re: [PATCH] documentation: clarify dirty_ratio and dirty_background

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Date:	Thu, 09 Oct 2008 09:32:03 +0200
From:	Andrea Righi <righi.andrea@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	Randy Dunlap <randy.dunlap@...cle.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Rubin <mrubin@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] documentation: clarify dirty_ratio and dirty_background_ratio
 description

KAMEZAWA Hiroyuki wrote:
> On Wed, 08 Oct 2008 12:52:28 +0200
> Andrea Righi <righi.andrea@...il.com> wrote:
> 
>> The current documentation of dirty_ratio and dirty_background_ratio is a
>> bit misleading.
>>
>> In the documentation we say that they are "a percentage of total system
>> memory", but the current page writeback policy, intead, is to apply the
>> percentages to the dirtyable memory, that means free pages + reclaimable
>> pages.
>>
> Right.
> 
>> Better to be more explicit to clarify this concept.
>>
>> Signed-off-by: Andrea Righi <righi.andrea@...il.com>
> 
> But I wonder "reclaimable memory" seems to be a difficult word for users....
> 
> "free pages + mapped pages + file cache, not including locked page and HugePage"
> ?
> Anyway,
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Sounds better. I'll add these details and post a new patch.

Thanks,
-Andrea

> 
>> ---
>>  Documentation/filesystems/proc.txt |   11 ++++++-----
>>  1 files changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
>> index f566ad9..be69c8b 100644
>> --- a/Documentation/filesystems/proc.txt
>> +++ b/Documentation/filesystems/proc.txt
>> @@ -1380,15 +1380,16 @@ causes the kernel to prefer to reclaim dentries and inodes.
>>  dirty_background_ratio
>>  ----------------------
>>  
>> -Contains, as a percentage of total system memory, the number of pages at which
>> -the pdflush background writeback daemon will start writing out dirty data.
>> +Contains, as a percentage of the dirtyable system memory (free pages +
>> +reclaimable pages), the number of pages at which the pdflush background
>> +writeback daemon will start writing out dirty data.
>>  
>>  dirty_ratio
>>  -----------------
>>  
>> -Contains, as a percentage of total system memory, the number of pages at which
>> -a process which is generating disk writes will itself start writing out dirty
>> -data.
>> +Contains, as a percentage of the dirtyable system memory (free pages +
>> +reclaimable pages), the number of pages at which a process which is generating
>> +disk writes will itself start writing out dirty data.
>>  
>>  dirty_writeback_centisecs
>>  -------------------------
>>
--
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