Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous fixes

[Paul E. McKenney]

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.  ;-)

							Thanx, Paul