linux-kernel - Re: [PATCH] Document huge memory/cache overhead of memory controller in Kconfig

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Message-ID: <47BC2275.4060900@linux.vnet.ibm.com>
Date:	Wed, 20 Feb 2008 18:22:05 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	akpm@...l.org, torvalds@...l.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] Document huge memory/cache overhead of memory controller
 in Kconfig

Andi Kleen wrote:
> Document huge memory/cache overhead of memory controller in Kconfig
> 
> I was a little surprised that 2.6.25-rc* increased struct page for the memory
> controller.  At least on many x86-64 machines it will not fit into a single
> cache line now anymore and also costs considerable amounts of RAM. 

The size of struct page earlier was 56 bytes on x86_64 and with 64 bytes it
won't fit into the cacheline anymore? Please also look at
http://lwn.net/Articles/234974/

> At earlier review I remembered asking for a external data structure for this.
> 
> It's also quite unobvious that a innocent looking Kconfig option with a 
> single line Kconfig description has such a negative effect.
> 
> This patch attempts to document these disadvantages at least so that users
> configuring their kernel can make a informed decision.
> 
> Cc: balbir@...ux.vnet.ibm.com
> 
> Signed-off-by: Andi Kleen <ak@...e.de>
> 
> Index: linux/init/Kconfig
> ===================================================================
> --- linux.orig/init/Kconfig
> +++ linux/init/Kconfig
> @@ -394,6 +394,14 @@ config CGROUP_MEM_CONT
>  	  Provides a memory controller that manages both page cache and
>  	  RSS memory.
> 
> +	  Note that setting this option increases fixed memory overhead
> +	  associated with each page of memory in the system by 4/8 bytes
> +	  and also increases cache misses because struct page on many 64bit
> +	  systems will not fit into a single cache line anymore.
> +
> +	  Only enable when you're ok with these trade offs and really
> +	  sure you need the memory controller.
> +

Looks good

Acked-by: Balbir Singh <balbir@...ux.vnet.ibm.com>

>  config PROC_PID_CPUSET
>  	bool "Include legacy /proc/<pid>/cpuset file"
>  	depends on CPUSETS


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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