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

[Davidlohr Bueso]

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 from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/