[systemd-devel] Allocating resource to achieve predictable run times

[Lennart Poettering]

On Mi, 19.06.19 09:25, John Lane (john at lane.uk.net) wrote:

> Well I got to the bottom of it and wanted to close this out. I was
> prototyping on a laptop and the cpu (i7-7820HQ) did funky things to save
> power. I tried setting the performance governor but it didn't help.
>
> I ran the exact same tests on some servers and got the expected
> behaviour on all of them.
>
> Regarding isolcpus, I did try that, although I noted from the kernel
> docs that it is a deprecated option and it suggests that cpuset should
> be used instead. But I don't believe systemd (yet) supports cpuset?

cpuset became a cgroups feature a long time ago, but it's really really awful
semanticswise in cgroupsv1. In cgroupsv2 this is being worked on and
should be fixed. As soon as that's done we'll support it.

That said, I don't really see how cpuset could ever provide what
isolcpus does, i.e. control what cpus the kernel's own threads are
scheduled on. I mean, cpuset is inherentlich an "a-posteriori" thing:
to schedule something on a cpu you need its pid, but kernel threads
come and go, and for those you really want to configure the cpuset
mask "a-priori"... So I must say I don't understand the comment in the
docs really, I don't see how cpuset and isolcpus could be equivalent.

Lennart

--
Lennart Poettering, Berlin