Re: VMA lookup with RCU

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Friday 05 October 2007 03:21, Peter Zijlstra wrote:
> On Thu, 2007-10-04 at 21:12 +0530, Vaidyanathan Srinivasan wrote:

> > Per CPU last vma cache:  Currently we have the last vma referenced in a
> > one entry cache in mm_struct.  Can we have this cache per CPU or per node
> > so that a multi threaded application can have node/cpu local cache of
> > last vma referenced.  This may reduce btree/rbtree traversal.  Let the
> > hardware cache maintain the corresponding VMA object and its coherency.
> >
> > Please let me know your comment and thoughts.
>
> Nick Piggin (and I think Eric Dumazet) had nice patches for this. I
> think they were posted in the private futex thread.

All they need is testing and some results to show they help. I actually
don't really have a realistic workload where vma lookup contention is
a problem, since the malloc fixes and private futexes went in.

Actually -- there is one thing, apparently oprofile does lots of find_vmas,
which trashes the vma cache. Either it should have its own cache, or at
least use a "nontemporal" lookup.

What I implemented was a per-thread cache. Per-CPU I guess would be
equally possible and might be preferable in some cases (although worse
in others). Still, the per-thread cache should be fine for basic performance
testing.

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