On Fri, 2014-02-21 at 10:13 -0800, Linus Torvalds wrote: > On Thu, Feb 20, 2014 at 9:28 PM, Davidlohr Bueso <davidlohr@xxxxxx> wrote: > > From: Davidlohr Bueso <davidlohr@xxxxxx> > > > > This patch is a continuation of efforts trying to optimize find_vma(), > > avoiding potentially expensive rbtree walks to locate a vma upon faults. > > Ok, so I like this one much better than the previous version. > > However, I do wonder if the per-mm vmacache is actually worth it. > Couldn't the per-thread one replace it entirely? 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. I originally left it there because I recall seeing a slightly better hit rate for some java workloads (map/reduce, specifically), but it wasn't a big deal - some slots endup being redundant with a per mm cache. It does however guarantee that we access hot vmas immediately, instead of potentially slightly more reads when we go into per-thread checking. I'm happy with the results either way. > 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. Thanks, Davidlohr -- 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>