Re: [PATCH] mm: cache largest vma

[Davidlohr Bueso]

On Fri, 2013-11-01 at 16:38 -0400, KOSAKI Motohiro wrote:
> (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.

Well I chose the largest vma because it gives us a greater chance of
being already cached when we do the lookup for the faulted address.

The 15% improvement was with Hadoop. According to my notes it was at
~48% with the baseline kernel and increased to ~63% with this patch.

In any case I didn't measure the rates on a per-task granularity, but at
a general system level. When a system is first booted I can see that the
mmap_cache access rate becomes the determinant factor and when adding a
workload it doesn't change much. One exception to this was a kernel
build, where we go from ~50% to ~89% hit rate on a vanilla kernel.

Thanks,
Davidlohr

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