linux-kernel - Re: non-NUMA cache_free_alien() (was Re: [RFC] SLAB : NUMA cache_free_alien() very expensive because of virt_to

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Date:	Thu, 22 Mar 2007 23:56:37 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
CC:	Christoph Lameter <christoph@...eter.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: non-NUMA cache_free_alien() (was Re: [RFC] SLAB : NUMA cache_free_alien()
 very expensive because of virt_to_slab(objp); nodeid = slabp->nodeid;)

Siddha, Suresh B a écrit :
> On Thu, Mar 22, 2007 at 11:12:39PM +0100, Eric Dumazet wrote:
>> Siddha, Suresh B a écrit :
>>> +	if (num_online_nodes() == 1)
>>> +		use_alien_caches = 0;
>>> +
>> Unfortunatly this part is wrong.
> 
> oops.
> 
>> You should check num_possible_nodes(), or nr_node_ids (this one is cheaper, 
>> its a variable instead of a function call)
> 
> But that is based on compile time option, isn't it? Perhaps I need
> to use some other mechanism to find out the platform is not NUMA capable..

nr_node_ids is defined to 1 if you compile a non NUMA kernel.

If CONFIG_NUMA is on, then nr_node_ids is a variable, that is filled with the 
maximum nodeid of possible node (+1). If your machine is not CPU hot plug 
capable, and you have say one node, (one dual core processor for example), 
then nr_node_ids will be set to 1
(see mm/page_alloc.c function setup_nr_node_ids() )

So this is OK for your need...

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