Re: 2.6.0 Huge pages not working as expected

[Linus Torvalds]

On Fri, 26 Dec 2003, Nick Craig-Wood wrote:
> 
> The results are just about the same - a slight slowdown for
> hugepages...

I don't think you are really testing the TLB - you are testing the data 
cache.

And the thing is, using huge pages will mean that the pages are 1:1
mapped, and thus get "perfectly" cache-coloured, while the anonymous mmap 
will give you random placement.

And what you are seeing is likely the fact that random placement is 
guaranteed to not have any worst-case behaviour. While perfect 
cache-coloring very much _does_ have worst-case schenarios, and you're 
likely triggering one of them.

In particular, using a pure power-of-two stride means that you are
limiting your cache to a certain subset of the full result with the
perfect coloring.

This, btw, is why I don't like page coloring: it does give nicely
reproducible results, but it does not necessarily improve performance.  
Random placement has a lot of advantages, one of which is a lot smoother
performance degradation - which I personally think is a good thing.

Try your program with non-power-of-two, and non-page-aligned strides. I
suspect the results will change (but I suspect that the TLB wins will 
still be pretty much in the noise compared to the actual data cache 
effects).

		Linus
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