Re: [PATCH] mm: cache largest vma

[Ingo Molnar <mingo@kernel.org>]

* Michel Lespinasse <walken@google.com> wrote:

> On Sun, Nov 10, 2013 at 8:12 PM, Davidlohr Bueso <davidlohr@hp.com> wrote:
> > 2) Oracle Data mining (4K pages)
> > +------------------------+----------+------------------+---------+
> > |    mmap_cache type     | hit-rate | cycles (billion) | stddev  |
> > +------------------------+----------+------------------+---------+
> > | no mmap_cache          | -        | 63.35            | 0.20207 |
> > | current mmap_cache     | 65.66%   | 19.55            | 0.35019 |
> > | mmap_cache+largest VMA | 71.53%   | 15.84            | 0.26764 |
> > | 4 element hash table   | 70.75%   | 15.90            | 0.25586 |
> > | per-thread mmap_cache  | 86.42%   | 11.57            | 0.29462 |
> > +------------------------+----------+------------------+---------+
> >
> > This workload sure makes the point of how much we can benefit of 
> > caching the vma, otherwise find_vma() can cost more than 220% extra 
> > cycles. We clearly win here by having a per-thread cache instead of 
> > per address space. I also tried the same workload with 2Mb hugepages 
> > and the results are much more closer to the kernel build, but with the 
> > per-thread vma still winning over the rest of the alternatives.
> >
> > All in all I think that we should probably have a per-thread vma 
> > cache. Please let me know if there is some other workload you'd like 
> > me to try out. If folks agree then I can cleanup the patch and send it 
> > out.
> 
> Per thread cache sounds interesting - with per-mm caches there is a real 
> risk that some modern threaded apps pay the cost of cache updates 
> without seeing much of the benefit. However, how do you cheaply handle 
> invalidations for the per thread cache ?

The cheapest way to handle that would be to have a generation counter for 
the mm and to couple cache validity to a specific value of that. 
'Invalidation' is then the free side effect of bumping the generation 
counter when a vma is removed/moved.

Thanks,

	Ingo