linux-kernel - Re: No, really, stop trying to delete slab until you've finished making slub perform as well

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Date:	Fri, 15 Aug 2008 13:24:24 -0500
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	Matthew Wilcox <matthew@....cx>,
	Pekka Enberg <penberg@...helsinki.fi>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Mel Gorman <mel@...net.ie>,
	andi@...stfloor.org, Rik van Riel <riel@...hat.com>
Subject: Re: No, really, stop trying to delete slab until you've finished
 making slub perform as well

KOSAKI Motohiro wrote:

>> Then we have another mechanism that will avoid
>> off node memory references by allocating new slabs. Can you set the
>> node_defrag parameter to 0? (Noted by Adrian).
> 
> Please let me know that operations ?

The control over the preferences of node local vs. remote defrag is occurring
via /sys/kernel/slab/<slabcache>/remote_node_defrag ratio. Default is 10%.
Comments in get_any_partial explain the operations.

The default setting means that in 9 out of 10 cases slub will prefer creating
a new slab over taking one from the remote node (meaning the memory is node
local, probably not important in your 2 node case). It will therefore waste
memory because local memory may be more efficient to use.

Setting remote_node_defrag_ratio to 100 will make slub always take the remote
slab instead of allocating a new one.


    /*
         * The defrag ratio allows a configuration of the tradeoffs between
         * inter node defragmentation and node local allocations. A lower
         * defrag_ratio increases the tendency to do local allocations
         * instead of attempting to obtain partial slabs from other nodes.
         *
         * If the defrag_ratio is set to 0 then kmalloc() always
         * returns node local objects. If the ratio is higher then kmalloc()
         * may return off node objects because partial slabs are obtained
         * from other nodes and filled up.
         *
         * If /sys/kernel/slab/xx/defrag_ratio is set to 100 (which makes
         * defrag_ratio = 1000) then every (well almost) allocation will
         * first attempt to defrag slab caches on other nodes. This means
         * scanning over all nodes to look for partial slabs which may be
         * expensive if we do it every time we are trying to find a slab
         * with available objects.
         */
        if (!s->remote_node_defrag_ratio ||
                        get_cycles() % 1024 > s->remote_node_defrag_ratio)
                return NULL;




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