linux-kernel - Re: adjusting lowmem_reserve_ratio on an embedded system?

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Message-Id: <20090702104227.1c07e85f.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 2 Jul 2009 10:42:27 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Daniel Kabs <daniel.kabs@....de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: adjusting lowmem_reserve_ratio on an embedded system?

On Wed, 1 Jul 2009 14:23:52 +0200
Daniel Kabs <daniel.kabs@....de> wrote:

> Hello,
> 
> in tracing a low on memory situation on an embedded system I came across 
> the Documentation/sysctl/vm.txt. There it is advised to change the 
> lowmem_reserve_ratio on systems running without swap.
> 
> I wonder if this applies for an embedded system running with 256MByte RAM 
> and no swap which is using only one memory zone (ZONE_NORMAL)? I guess 
> no.
> 
> 
> To understand what the parameter means I read the relevant section from 
> vm.txt and came across the terms "lowmem zone" and "lower zones" 
> and "lowmem memory". I feel dumb. I could not find a concept of "lower 
> zones" (or "upper" zones).  What exactly do they mean?
> 
Assume a system with HIGHMEM/NORMAL/DMA

zone fallback list is created as

	HIGHMEM->NORMAL->DMA

HIGHMEM's lower zone is NORMAL+DMA
NORMAL's lower zone is DMA.

"lowmem" often means NORMAL+DMA (yes, there are HIGHMEM) but it depends on context.

Regards,
-Kame

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