Purpose of the mm/slab.c changes

Manfred Spraul (manfred@colorfullife.com)
Sun, 09 Sep 2001 13:05:34 +0200

What's the purpose of the mm/slab.c changes?

Linus, Alan, could you please drop them. Switching from 1 list to 3
lists is just a slowdown.

2.4.9 2.4.9-ac9
km(1) 0x87 0x89
kf(1) 0xa9 0xa8
kf(cachep, 1) 0xad 0xad
km(4096) 0x7B 0x87 (+24% without overhead!)
kf(4096) 0xcB 0xd1
kf(cachep,4096) 0xcc 0xd3

(cpu ticks on a Duron 700, UP kernel, 100 calls in a tight loop, loop
overhead (0x49) not removed)

Benchmarking with SMP kernel is meaningless, since the actual list
changes are outside of the hot path - I did it, and the differences are
negligable (+-1 cpu tick)

And this is a benchmark with 100 calls in a tight loop - Andrea's patch
adds several additional branches, in real-world there would be an even
larger slowdown.

If there are real performance problems with the slab, then the not-yet
optimized parts should get changed:

* kmalloc(): the loop to find the correct slab is too slow. (the numbers
in the table are without the loop, i.e.
kmem_cache_alloc(kmem_find_general_cache(4096, SLAB_KERNEL),

* finetune the size of the per-cpu caches
* finetune the amount of pages freed during kmem_cache_reap()
* replace the division in kmem_cache_free_one() with a multiplication.
(UP only, on SMP outside of the hot path)
* enable OPTIMIZE - could help on platforms without a fast
virt_addr->struct page lookup.

I have a patch with some optimization, but I tought that would be 2.5


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