Re: large page patch

From: Linus Torvalds (
Date: Thu Aug 01 2002 - 23:32:44 EST

On Thu, 1 Aug 2002, David S. Miller wrote:
> Of course, if you can actually measure it, that would be
> interesting. Naive math gives you a guess for the order of
> magnitude effect, but nothing beats real numbers ;)
> The SYSV folks actually did have a buddy allocator a long time ago and
> they did implement lazy coalescing because is supposedly improved
> performance.

I bet that is mainly because of CPU scalability, and being able to avoid
touching the buddy lists from multiple CPU's - the same reason _we_ have
the per-CPU front-ends on various allocators.

I doubt it is because buddy matters past the 4MB mark. I just can't see
how you can avoid the naive math which says that it should be 1/512th as
common to coalesce to 4MB as it is to coalesce to 8kB.

Walking the buddy bitmaps for a few levels (ie up to order 3 or 4) is
probably quite common, and it's likely to be bad from a SMP cache
standpoint (touching a few bits with what must be fairly random patterns).
So avoiding the buddy with a simple front-end is likely to win you
something, without actually being meaningful at the MAX_ORDER point.


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