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Subject: Re: [PATCH] ext4: slab caches set to SLAB_MEM_SPREAD flags.
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From: Theodore Tso <tytso@MIT.EDU>
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Date: Wed, 16 Nov 2011 10:51:59 -0500
Cc: Theodore Tso <tytso@MIT.EDU>, linux-kernel@vger.kernel.org,
        linux-ext4@vger.kernel.org, Amit Sahrawat <amit.sahrawat83@gmail.com>
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To: Namjae Jeon <linkinjeon@gmail.com>
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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.

For the pageio, they are used immediately, and may end up getting freed on the same CPU if the I/O interrupt (and hence competion callbacks) happen on the same CPU that they are submitted on….

Regards,

-- Ted