RT/Linux SCHED_RR/_FIXED to combat latency?

[Nico Kadel-Garcia]

On Wed, Aug 9, 2023 at 10:42 PM Cedric Blancher
<cedric.blancher at gmail.com> wrote:
>
> Good morning!
>
> We're experiencing rather very bad latency spikes on busy Linux
> systems, for example if one machine is the jumphost (ssh -J) for a few
> hundred connections, while at the same time handles CPU intensive
> tasks.
>
> Would RT/Linux SCHED_FIXED or SCHED_RR be of help in such a case, e.g.
> put all ssh processes into the SCHED_FIXED scheduling class, with a
> priority higher than the non-interactive compute processes?

Real Time Linux doesn't solve these problems. It attempts to handle
the variety of interrupts more consistently and equitably, but
precisely the sort of "I'm too danged busy with this high priority
process" issues of a highly burdened server are likely to be
*aggraveted* by the incorrect guesses of what processes really matter
on a "real-time" system.

If you know which problems are most important and can raise their
priority, great, but unpredictable delays are usually the sign of a
"too-busy with too many processes" kernel.