On Mon, Jan 29, 2024 at 11:43:08PM +0100, Frederic Weisbecker wrote:
> 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.
Some IBM clients made use of nohz_full for HPC workloads, and claimed
to see significant benefits. (Single-digit percents, if I remember
correctly, but still...)
Thanx, Paul
