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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Feb 20, 2014 at 9:28 PM, Davidlohr Bueso <davidlohr@hp.com> wrote:
> From: Davidlohr Bueso <davidlohr@hp.com>
>
> 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?

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.

What am I missing?

               Linus

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>