RE: [PATCH v2] eal/unix: allow creating thread with real-time priority

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> Sent: Thursday, 26 October 2023 18.32
> 
> On Thu, 26 Oct 2023 09:33:42 +0200
> Morten Brørup <mb@smartsharesystems.com> wrote:
> 
> > > From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> > > Sent: Wednesday, 25 October 2023 23.33
> > >
> > > On Wed, 25 Oct 2023 19:54:06 +0200
> > > Morten Brørup <mb@smartsharesystems.com> wrote:

[...]

> > > > Someone might build a kernel with options to keep non-dataplane
> threads off
> > > some dedicated CPU cores, so they can be used for guaranteed low-
> latency
> > > dataplane threads. We do. We don't use real-time priority, though.
> > >
> > > This is really, hard to do.
> >
> > As my kids would say: This is really, really, really, really, really
> hard to do!
> >
> > We have not been able to find an authoritative source of
> documentation describing how to do it. :-(

[...]

> One benefit of doing real-time thread is that kernel will be more
> precise in
> any calls to sleep. If you do small sleep in normal thread, the kernel
> will round
> up the timer to try and avoid reprogramming timer chip and to save
> power (less wakeups from idle).
> With RT thread it will do "you wanted 21us, ok for you will do 21us"

So we don't need PR_SET_TIMERSLACK with RT threads?

> 
> The project that was originally Vyatta, has a script that tries to
> isolate interrupts etc.
> I started it but they have worked on it since then.
> 
>    https://github.com/danos/vyatta-cpu-shield
> 
> It adjust kernel workers, softirq, cgroups etc

This script looks very interesting. Thank you, Stephen!