Re: SLUB defrag pull request?

From: Christoph Lameter
Date: Thu Oct 23 2008 - 10:26:29 EST

On Thu, 23 Oct 2008, Pekka Enberg wrote:

The problem looks like its freeing objects on a different processor that
where it was used last. With the pointer array it is only necessary to touch
the objects that contain the arrays.

Interesting. SLAB gets away with this because of per-cpu caches or
because it uses the bufctls instead of a freelist?

Exactly. Slab adds a special management structure to each slab page that contains the freelist and other stuff. Freeing first occurs to a per cpu queue that contains an array of pointers. Then later the objects are moved from the pointer array into the management structure for the slab.

What we could do for SLUB is to generate a linked list of pointer arrays in the free objects of a slab page. If all objects are allocated then no pointer array is needed. The first object freed would become the first pointer array. If that is found to be exhausted then the object currently being freed is becoming the next pointer array and we put a link to the old one into the object as well.

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