Re: [PATCH] mm: per-thread vma caching

[Ingo Molnar <mingo@kernel.org>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Fri, Feb 21, 2014 at 12:53 PM, Davidlohr Bueso <davidlohr@hp.com> wrote:
> >
> > I think you are right. I just reran some of the tests and things are
> > pretty much the same, so we could get rid of it.
> 
> Ok, I'd prefer the simpler model of just a single per-thread hashed
> lookup, and then we could perhaps try something more complex if there
> are particular loads that really matter. I suspect there is more
> upside to playing with the hashing of the per-thread cache (making it
> three bits, whatever) than with some global thing.
> 
> >> Also, the hash you use for the vmacache index is *particularly* odd.
> >>
> >>         int idx =  (addr >> 10) & 3;
> >>
> >> you're using the top two bits of the address *within* the page.
> >> There's a lot of places that round addresses down to pages, and in
> >> general it just looks really odd to use an offset within a page as an
> >> index, since in some patterns (linear accesses, whatever), the page
> >> faults will always be to the beginning of the page, so index 0 ends up
> >> being special.
> >
> > Ah, this comes from tediously looking at access patterns. I actually
> > printed pages of them. I agree that it is weird, and I'm by no means
> > against changing it. However, the results are just too good, specially
> > for ebizzy, so I decided to keep it, at least for now. I am open to
> > alternatives.
> 
> Hmm. Numbers talk, bullshit walks. So if you have the numbers that say
> this is actually a good model..
> 
> I guess that for any particular page, only the first access address
> matters. And if it really is a "somewhat linear", and the first access
> tends to hit in the first part of the page, and the cache index tends
> to cluster towards idx=0. And for linear accesses, I guess *any*
> clustering is actually a good thing, since spreading things out just
> defeats the fact that linear accesses also tend to hit in the same
> vma.
> 
> And if you have truly fairly random accesses, then presumably their
> offsets within the page are fairly random too, and so hashing by
> offset within page might work well to spread out the vma cache
> lookups.
> 
> So I guess I can rationalize it. [...]

Davidlohr: it would be nice to stick a comment about the (post facto) 
rationale into the changelog or the code (or both).

Thanks,

	Ingo

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