Re: [Xen-devel] [PATCH v5 20/19] docs: add "sched-gran" boot parameter documentation

[Jürgen Groß <jgross@xxxxxxxx>]

On 30.09.19 12:25, Jan Beulich wrote:
> On 30.09.2019 12:09, Juergen Gross wrote:
> > Add documentation for the new "sched-gran" hypervisor boot parameter.
> >
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> >   docs/misc/xen-command-line.pandoc | 21 +++++++++++++++++++++
> >   1 file changed, 21 insertions(+)
> >
> > diff --git a/docs/misc/xen-command-line.pandoc 
> > b/docs/misc/xen-command-line.pandoc
> > index fc64429064..c855246050 100644
> > --- a/docs/misc/xen-command-line.pandoc
> > +++ b/docs/misc/xen-command-line.pandoc
> > @@ -1782,6 +1782,27 @@ Set the timeslice of the credit1 scheduler, in 
> > milliseconds.  The
> >   default is 30ms.  Reasonable values may include 10, 5, or even 1 for
> >   very latency-sensitive workloads.
> >   
> > +### sched-gran (x86)
> > +> `= cpu | core | socket`
> > +
> > +> Default: `sched-gran=cpu`
> > +
> > +Set the scheduling granularity. In case the granularity is larger than 1 (e.g.
> > +`core`on a SMT-enabled system, or `socket`) multiple vcpus are assigned
> > +statically to a "scheduling unit" which will then be subject to scheduling.
> > +This assignment of vcpus to scheduling units is fixed.
> > +
> > +`cpu`: Vcpus will be scheduled individually on single cpus.
> > +
> > +`core`: As many vcpus as there are hyperthreads on a physical core are
> > +scheduled together on a physical core.
> > +
> > +`socket`: As many vcpus as there are hyperthreads on a physical sockets are
> > +scheduled together on a physical socket.
> I'd prefer if this didn't end up Intel-centric; ideally it also wouldn't be
> x86-specific. AMD has introduced hyperthreading in Fam17 only; Fam15 used
> "compute units", grouping together "cores". Internally the Intel side
> "core vs hyperthread" is represented in the same variables (cpu_sibling_mask
> in particular) as the AMD side "compute unit vs core".
Yes, it is a mess.

> Therefore it may be better to talk here about e.g. "smallest topological
> sub-unit" and only say "e.g. a hyperthread to make a connection to common
> x86 / Intel terminology". Of course the AMD side alternative use of the
> variables also renders the actual command line option "sched-gran=core"
> not overly fortunate. Perhaps we'd want to also use more abstract terms
> here, e.g. topological "levels"?
I think regarding usage of "hyperthreads" I'll go with:

+`cpu`: Vcpus will be scheduled individually on single cpus (e.g. a
+ hyperthread using x86/Intel terminology)
+
+ `core`: As many vcpus as there are cpus on a physical core are
+ scheduled together on a physical core.
...

I think using "core" is fine. We have it in multiple places in the
hypervisor which are _not_ specific to Intel. And "core-scheduling" is
a well-known buzzword already.

> > +Note: a value other than `cpu` will result in rejecting a runtime modification
> > +of the "smt" setting.
> Perhaps add "attempt" here?
Yes.


Juergen

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