* Davidlohr Bueso <davidlohr@xxxxxx> wrote: > On Sun, 2013-11-03 at 11:12 +0100, Ingo Molnar wrote: > > * Davidlohr Bueso <davidlohr@xxxxxx> 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. > > > > > > This patch introduces a second mmap_cache pointer, which is just > > > as racy as the first, but as we already know, doesn't matter in > > > this context. For documentation purposes, I have also added the > > > ACCESS_ONCE() around mm->mmap_cache updates, keeping it consistent > > > with the reads. > > > > > > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > > > Cc: Michel Lespinasse <walken@xxxxxxxxxx> > > > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > > > Cc: Mel Gorman <mgorman@xxxxxxx> > > > Cc: Rik van Riel <riel@xxxxxxxxxx> > > > Cc: Guan Xuetao <gxt@xxxxxxxxxxxxxxx> > > > Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx> > > > --- > > > Please note that nommu and unicore32 arch are *untested*. > > > > > > I also have a patch on top of this one that caches the most > > > used vma, which adds another 8-10% hit rate gain, However, > > > since it does add a counter to the vma structure and we have > > > to do more logic in find_vma to keep track, I was hesitant about > > > the overhead. If folks are interested I can send that out as well. > > > > Would be interesting to see. > > > > Btw., roughly how many cycles/instructions do we save by increasing > > the hit rate, in the typical case (for example during a kernel build)? > > Good point. The IPC from perf stat doesn't show any difference with or > without the patch -- note that this is probably the least interesting > one as we already get a really nice hit rate with the single mmap_cache. > I have yet to try it on the other workloads. I'd be surprised if this was measureable via perf stat, unless you do the measurement in a really, really careful way - and even then it's easy to make a hard to detect mistake larger in magnitude than the measured effect ... An easier and more reliable measurement would be to stick 2-3 get_cycles() calls into the affected code and save the pure timestamps into task.se.statistics, and extract the timestamps via /proc/sched_debug by adding matching seq_printf()s to kernel/sched/debug.c. (You can clear the statistics by echoing 0 to /proc/<PID>/sched_debug, see proc_sched_set_task().) That measurement is still subject to skid and other artifacts but hopefully the effect is larger than cycles fuzz - and we are interested in a ballpark figure in any case. Thanks, Ingo -- 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>