Re: Observation on NOHZ_FULL

[Andrea Righi <andrea.righi@canonical.com>]

On Thu, Feb 08, 2024 at 12:54:58AM -0500, Joel Fernandes wrote:
...
> >> Slightly related, but one of the things we are wondering also is how
> >> much of the overhead for your nohz-full and lazy-RCU test (on top of
> >> baseline - that is just CONFIG_HZ=1000 without nohz-full or nocbs) is
> >> because of just using NOCB. Uladsizlau mentioned he might run a test
> >> for comparing along those lines as well.
> > 
> > Just to clarify, "lazy rcu on" results are just with rcu_nocb=all and
> > lazy RCUs enabled (and HZ=1000), so without nohz_full.
> > 
> > If I enable only nohz_full=all (without rcu_nocb) I see something like
> > this:
> 
> Ok. I did want to mention nohz_full implies rcu_nocb on the same CPUs as well.
> 
> Its also mentioned in the boot param docs on the last line of the description:
> 
>         nohz_full=      [KNL,BOOT,SMP,ISOL]
>                         The argument is a cpu list, as described above.
>                         In kernels built with CONFIG_NO_HZ_FULL=y, set
>                         the specified list of CPUs whose tick will be stopped
>                         whenever possible. The boot CPU will be forced outside
>                         the range to maintain the timekeeping.  Any CPUs
>                         in this list will have their RCU callbacks offloaded,
>                         just as if they had also been called out in the
>                         rcu_nocbs= boot parameter.

Ah I didn't realize that, it definitely makes sense, thanks for
clarifying it.

Then basically in the results that I posted the difference is
"nohz_full=all+rcu_nocb=all" vs "rcu_nocb=all+lazy_RCU=on".

> 
> (btw, I am a bit slow to respond to email due to being ill).

Get well soon!

> 
> thanks,
> 
>  - Joel

-Andrea