Re: slab-alignment-rework.patch in -mc

From: Manfred Spraul
Date: Mon Apr 19 2004 - 11:48:36 EST

Andrea Arcangeli wrote:


I don't think this is right:

- if (flags & SLAB_HWCACHE_ALIGN) {
- /* Need to adjust size so that objs are cache aligned. */
- /* Small obj size, can get at least two per cache line. */
+ if (!align) {
+ /* Default alignment: compile time specified l1 cache size.
+ * Except if an object is really small, then squeeze multiple
+ * into one cacheline.
+ */
+ align = cache_line_size();
while (size <= align/2)
align /= 2;
- size = (size+align-1)&(~(align-1));
+ size = ALIGN(size, align);

I want anon-vma to really use only 12 bytes, period.

Then pass "4" as the align parameter to kmem_cache_create. That's the main point of the patch: it's now possible to explicitely specify the requested alignment. 32 for the 3rd level page tables, the optimal number for the pte_chains, etc.

No best-guess must
be made automatically by the slab code, rounding it to 16 bytes.

If you pass 0 as align to kmem_cache_create, then it's rounded to L2 size. It's questionable if that's really the best thing - on uniprocessor, 16-byte might result is better performance - there is no risk of false sharing.


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