Re: [PATCH] mm: cache largest vma

[Davidlohr Bueso]

On Mon, 2013-11-11 at 13:04 +0100, Ingo Molnar wrote:
> * Michel Lespinasse <walken@xxxxxxxxxx> wrote:
> 
> > On Sun, Nov 10, 2013 at 8:12 PM, Davidlohr Bueso <davidlohr@xxxxxx> 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.

I was basing the invalidations on the freeing of vm_area_cachep, so I
mark current->mmap_cache = NULL whenever we call
kmem_cache_free(vm_area_cachep, ...). But I can see this being a problem
if more than one task's mmap_cache points to the same vma, as we end up
invalidating only one. I'd really like to use a similar logic and base
everything around the existence of the vma instead of adding a counting
infrastructure. Sure we'd end up doing more reads when we do the lookup
in find_vma() but the cost of maintaining it comes free. I just ran into
a similar idea from 2 years ago:
http://lkml.indiana.edu/hypermail/linux/kernel/1112.1/01352.html

While there are several things that aren't needed, it does do the
is_kmem_cache() to verify that the vma is still a valid slab.

Thanks,
Davidlohr


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