On Mon, Aug 29, 2022 at 4:42 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Mon, Aug 29, 2022 at 04:36:40PM -0400, Joel Fernandes wrote: > > On Mon, Aug 29, 2022 at 3:46 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote: > > > On Mon, Aug 29, 2022 at 12:45:40PM -0400, Joel Fernandes wrote: > > > > On 8/29/2022 9:40 AM, Frederic Weisbecker wrote: > > [ . . . ] > > > > > > 2) NOCB implies performance issues. > > > > > > > > Which kinds of? There is slightly worse boot times, but I'm guessing that's do > > > > with the extra scheduling overhead of the extra threads which is usually not a > > > > problem except that RCU is used in the critical path of boot up (on ChromeOS). > > > > > > I never measured it myself but executing callbacks on another CPUs, with > > > context switches and locking can only involve significant performance issues if callbacks > > > are frequent. So it's a tradeoff between power and performance. > > > > In my testing of benchmarks on real systems with 8-16 CPUs, the > > performance hit is down in the noise. It is possible though that maybe > > one can write a non-realistic synthetic test to force the performance > > issues, but I've not seen it in the real world. Maybe on > > networking-heavy servers with lots of cores, you'll see it but their > > batteries if any would be pretty big :-). > > To Frederic's point, if you have enough servers, even a 1% decrease in > power consumption is a very big deal. ;-) Ah I see Frederick's point now, so basically the claim is that using lazy-RCU on servers might make sense to save power because CONFIG_RCU_NO_CB_CPU may not be an option there (networking throughput and so forth). That's a good point indeed... As you said, let us see v5 and how we want to proceed from there (as it is not too far from posting) . I do appreciate Frederick's review and valid concerns. Thanks, - Joel
