linux-kernel - Re: LowFree/LowMem problem

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Date:	Fri, 25 Jan 2008 02:45:34 +0100
From:	Matthias Wolle <Matthias.Wolle@....de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: LowFree/LowMem problem

On Thursday 24 January 2008 08:16 Andi Kleen wrote:
> Do you perhaps have a kernel compiled without CONFIG_HIGHPTE? Normally
> __pte_alloc should be able to allocate highmem unless that option is
> not set. Before HIGHPTE was implemented running out of low memory
> due to page tables was pretty common.

The kernel from 17th Jan 2008 didn't have CONFIG_HIGHPTE enabled. Yesterday we 
checked a 2.6.23.14 kernel with CONFIG_HIGHPTE enabled. The 4GB test machine 
showed the same fast low memory reducing effect. LowFree stopped at 400MB 
like without CONFIG_HIGHPTE.  A 2GB machine had a limit of 690MB LowFree. The 
difference of LowTotal and LowFree is shown as used memory. In case of the 
4GB machine, this means I have 400MB used memory which is not related to any 
process. 
I hope this helps. I can't test on the 16GB RAM machines anymore, because they 
are working productive now.  

> BTW the ultimate fix for most lowmem problems is to go 64bit

We switched back to the distribution default 2.6.18-bigmem kernel of debian 
which doesn't have this problem. On this kernel the test script has no effect 
to the low memory. 

Regards
Matthias

On Thursday 24 January 2008 08:16 Andi Kleen wrote:
> Matthias Wolle <Matthias.Wolle@....de> writes:
> > Jan 17 23:31:58 franklin72 kernel: sshd invoked oom-killer:
> > gfp_mask=0xd0, order=0, oomkilladj=0 Jan 17 23:31:58 franklin72 kernel:
> > cat invoked oom-killer: gfp_mask=0x84d0, order=0, oomkilladj=0 Jan 17
> > 23:31:58 franklin72 kernel:  [<c014af39>] out_of_memory+0x69/0x1a4 Jan 17
> > 23:31:58 franklin72 kernel:  [<c014c588>] __alloc_pages+0x20a/0x291 Jan
> > 17 23:31:58 franklin72 kernel:  [<c0153618>] __pte_alloc+0x11/0x94
>
> Do you perhaps have a kernel compiled without CONFIG_HIGHPTE? Normally
> __pte_alloc should be able to allocate highmem unless that option is
> not set. Before HIGHPTE was implemented running out of low memory
> due to page tables was pretty common.
>
> BTW the ultimate fix for most lowmem problems is to go 64bit
>
> -Andi
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