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 23:12:39 +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 :
> Christoph,
> 
> While we are at this topic, recently I had reports that
> cache_free_alien() is costly on non NUMA platforms too (similar
> to the cache miss issues that Eric was referring to on NUMA)
> and the appended patch seems to fix it for non NUMA atleast.
> 
> Appended patch gives a nice 1% perf improvement on non-NUMA platform
> with database workload.
> 
> Please comment or Ack for mainline :)

I have one comment :)

> @@ -1394,6 +1394,9 @@ void __init kmem_cache_init(void)
>  	int order;
>  	int node;
>  
> +	if (num_online_nodes() == 1)
> +		use_alien_caches = 0;
> +

Unfortunatly this part is wrong.

You should check num_possible_nodes(), or nr_node_ids (this one is cheaper, 
its a variable instead of a function call)

I wonder if we could add a new SLAB_NUMA_BYPASS, so that we can declare some 
kmem_cache as non NUMA aware (for example, I feel network skb dont need the 
NUMA overhead)


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