On Thu, 10 Apr 2014 14:30:03 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > On Thu, Apr 10, 2014 at 03:17:41PM -0400, Steven Rostedt wrote: > > On Thu, 10 Apr 2014 17:36:17 +0200 > > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > > > > It defaults to the total number of CPUs in the system, given the default > > > setup (all CPUs in a single balance domain), this should result in all > > > CPUs working concurrently on the boosted read sides. > > > > Unfortunately, it currently defaults to the number of possible CPUs in > > the system. I should probably move the default assignment to after SMP > > is setup. Currently it happens in early boot before all the CPUs are > > running. On boot up, the limit is set to NR_CPUS which should be much > > higher than what the system has, but shouldn't matter during boot. But > > after all the CPUs are up and running, it can lower it to online CPUs. > > Another approach is to use nr_cpu_ids, which is the maximum number of > CPUs that the particular booting system could ever have. I use this in > RCU to resize the data structures down from their NR_CPUS compile-time > hugeness. > OK, also, in doing our benchmarks, there's a big difference with rt_rw_limit being num_online_cpus and 2 * num_online_cpus. It doesn't seem to get better adding more than that. This was shown on a case with 12 cpus as well as 8 cpus. Same result. I really like to see a real use case benefit to find the best default. But as our mmap_sem stress test shows 2xCPUS as being the best, I'm going to go with that until someone comes up with a better test. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
