Re: [PATCH] mm: cache largest vma

From: KOSAKI Motohiro
Date: Fri Nov 01 2013 - 16:38:43 EST

(11/1/13 4:17 PM), Davidlohr Bueso wrote:
While caching the last used vma already does a nice job avoiding
having to iterate the rbtree in find_vma, we can improve. After
studying the hit rate on a load of workloads and environments,
it was seen that it was around 45-50% - constant for a standard
desktop system (gnome3 + evolution + firefox + a few xterms),
and multiple java related workloads (including Hadoop/terasort),
and aim7, which indicates it's better than the 35% value documented
in the code.

By also caching the largest vma, that is, the one that contains
most addresses, there is a steady 10-15% hit rate gain, putting
it above the 60% region. This improvement comes at a very low
overhead for a miss. Furthermore, systems with !CONFIG_MMU keep
the current logic.

I'm slightly surprised this cache makes 15% hit. Which application
get a benefit? You listed a lot of applications, but I'm not sure
which is highly depending on largest vma.

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