Re: [RFC] Simple Slab: A slab allocator with minimal meta information

From: Manfred Spraul
Date: Fri Aug 11 2006 - 16:31:42 EST

Christoph Lameter wrote:

I still do not get the role of the shared cache though.

The shared cache is just for efficient object transfers:
Think about two nics, both cpu bound, one does rx, the other does tx.
Result: a few 100k kmalloc, kmem_cache_alloc(skb_head_cache) calls each second on cpu1.
the same number of kfree, kmem_cache_free(skb_head_cache) calls each second on cpu 2.

What is needed is an efficient algorithm for transfering all objects from cpu 2 to cpu 1.
Initially, the slab allocator just had the cpu cache. Thus an object transfer was a free_block call: add the freed object to the bufctl linked list. Move the slab to the tail of the partial list. Probably the list_del()/list_add() calls caused cache line trashing, but I don't remember the details. IIRC Robert Olsson did the test. Pentium III Xeon system?
Anyway: The solution was the shared array. It allows to move objects around with a simple memmove of the pointers, without the list_del()/list_add() calls.

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