* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Fri, Feb 21, 2014 at 12:53 PM, Davidlohr Bueso <davidlohr@xxxxxx> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>