On Mon, 2014-03-03 at 16:00 -0800, Andrew Morton wrote: > On Thu, 27 Feb 2014 13:48:24 -0800 Davidlohr Bueso <davidlohr@xxxxxx> wrote: > > > From: Davidlohr Bueso <davidlohr@xxxxxx> > > > > This patch is a continuation of efforts trying to optimize find_vma(), > > avoiding potentially expensive rbtree walks to locate a vma upon faults. > > The original approach (https://lkml.org/lkml/2013/11/1/410), where the > > largest vma was also cached, ended up being too specific and random, thus > > further comparison with other approaches were needed. There are two things > > to consider when dealing with this, the cache hit rate and the latency of > > find_vma(). Improving the hit-rate does not necessarily translate in finding > > the vma any faster, as the overhead of any fancy caching schemes can be too > > high to consider. > > > > We currently cache the last used vma for the whole address space, which > > provides a nice optimization, reducing the total cycles in find_vma() by up > > to 250%, for workloads with good locality. On the other hand, this simple > > scheme is pretty much useless for workloads with poor locality. Analyzing > > ebizzy runs shows that, no matter how many threads are running, the > > mmap_cache hit rate is less than 2%, and in many situations below 1%. > > > > The proposed approach is to replace this scheme with a small per-thread cache, > > maximizing hit rates at a very low maintenance cost. Invalidations are > > performed by simply bumping up a 32-bit sequence number. The only expensive > > operation is in the rare case of a seq number overflow, where all caches that > > share the same address space are flushed. Upon a miss, the proposed replacement > > policy is based on the page number that contains the virtual address in > > question. Concretely, the following results are seen on an 80 core, 8 socket > > x86-64 box: > > > > ... > > > > 2) Kernel build: This one is already pretty good with the current approach > > as we're dealing with good locality. > > > > +----------------+----------+------------------+ > > | caching scheme | hit-rate | cycles (billion) | > > +----------------+----------+------------------+ > > | baseline | 75.28% | 11.03 | > > | patched | 88.09% | 9.31 | > > +----------------+----------+------------------+ > > What is the "cycles" number here? I'd like to believe we sped up kernel > builds by 10% ;) > > Were any overall run time improvements observable? Weeell not too much (I wouldn't normally go measuring cycles if I could use a benchmark instead ;). As discussed a while back, all this occurs under the mmap_sem anyway, so while we do optimize find_vma() in more workloads than before, it doesn't translate in better benchmark throughput :( The same occurs if we get rid of any caching and just rely on rbtree walks, sure the cost of find_vma() goes way up, but that really doesn't hurt from a user perspective. Fwiw, I did see in ebizzy perf traces find_vma goes from ~7% to ~0.4%. > > > ... > > > > @@ -1228,6 +1229,9 @@ struct task_struct { > > #ifdef CONFIG_COMPAT_BRK > > unsigned brk_randomized:1; > > #endif > > + /* per-thread vma caching */ > > + u32 vmacache_seqnum; > > + struct vm_area_struct *vmacache[VMACACHE_SIZE]; > > So these are implicitly locked by being per-thread. Yes. > > +static inline void vmacache_invalidate(struct mm_struct *mm) > > +{ > > + mm->vmacache_seqnum++; > > + > > + /* deal with overflows */ > > + if (unlikely(mm->vmacache_seqnum == 0)) > > + vmacache_flush_all(mm); > > +} > > What's the locking rule for mm->vmacache_seqnum? Invalidations occur under the mmap_sem (writing), just like mm->mmap_cache did. 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>