Re: large page patch

[Linus Torvalds]

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.

		Linus