linux-ext4 - Re: [PATCH] ext4: slab caches set to SLAB_MEM

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Date:	Wed, 16 Nov 2011 13:52:46 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Theodore Tso <tytso@....edu>
cc:	Namjae Jeon <linkinjeon@...il.com>, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org,
	Amit Sahrawat <amit.sahrawat83@...il.com>
Subject: Re: [PATCH] ext4: slab caches set to SLAB_MEM_SPREAD flags.

On Wed, 16 Nov 2011, Theodore Tso wrote:

> On Nov 16, 2011, at 10:04 AM, Namjae Jeon wrote:
> 
> > If slab caches set to SLAB_MEM_SPREAD flags, The allocation is spread 
> > evenly over all the memory nodes instead of favoring allocation on the 
> > node local to current cpu.
> 
> And why do you think this is a good thing?   For mballoc in particular, 
> the data structures are used immediately and then freed immediately --- 
> on the local node, so using a non-local memory just makes things worse 
> in a NUMA system.
> 

I don't think this has the effect that Namjae thinks it does: this is only 
useful for CONFIG_SLAB and when you have cpusets enabled with 
cpuset.memory_spread_slab set.

To test how useful it is, you should enable CONFIG_SLAB and then mount 
cpusets, set cpuset.memory_spread_slab, and create an MPOL_INTERLEAVE 
mempolicy over all online nodes.  This will have the same effect as adding 
SLAB_MEM_SPREAD to these slab caches (it just doesn't require the 
mempolicy) and will be able to quantify the effects without any changes to 
the kernel at all.
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