Re: Observation on NOHZ_FULL

[Frederic Weisbecker <frederic@kernel.org>]

Le Mon, Jan 29, 2024 at 05:20:23PM -0500, Joel Fernandes a écrit :
> > If i do not miss something
> > the NO_HZ_FULL will disable the timer if there is only one task on CPU
> > so that running task benefits from not being interrupted thus gets more
> > CPU time.
> 
> Yes, that's right. I believe it is well known that HPC-type of workloads benefit
> from FULL, however it has led to want to try it out for constrained system as
> well where CPU cycles are a premium, especially if the improvement is like what
> the report suggests (give or take the concerns/questions Paul raised).

I'll be unable to suggest anything related to that Bogomips calculation but
I must add something about HPC.

I have long believed that HPC would benefit from nohz_full but I actually never
heard of any user of that. The current known users of nohz_full are workloads
that don't use the kernel once the application is launched and do their own
stack of, for example, networking, talking directly to the device from
userspace. Using DPDK for example. These usecases are for extremely low latency
expectations (a single interrupt can make you lose).

HPC looks to me different, making use of syscalls and kernel for I/O. Nohz_full
may remove timer IRQs but it adds performance loss on kernel entry, making it
probably unsuitable there. But I might be wrong.

Thanks.

> 
> Thanks,
> 
>  - Joel
>