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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>