Re: [RFC PATCH v4 00/19] Core scheduling v4

Aubrey Li
2020-02-27T14:37:38Z

On Thu, Feb 27, 2020 at 10:10 PM Phil Auld <pauld@redhat.com> wrote:
>
> Hi Aaron,
>
> On Thu, Feb 27, 2020 at 10:04:32AM +0800 Aaron Lu wrote:
> > On Tue, Feb 25, 2020 at 03:51:37PM -0500, Vineeth Remanan Pillai wrote:
> > > On a 2sockets/16cores/32threads VM, I grouped 8 sysbench(cpu mode)
> > > > threads into one cgroup(cgA) and another 16 sysbench(cpu mode) threads
> > > > into another cgroup(cgB). cgA and cgB's cpusets are set to the same
> > > > socket's 8 cores/16 CPUs and cgA's cpu.shares is set to 10240 while cgB's
> > > > cpu.shares is set to 2(so consider cgB as noise workload and cgA as
> > > > the real workload).
> > > >
> > > > I had expected cgA to occupy 8 cpus(with each cpu on a different core)
> > >
> > > The expected behaviour could also be that 8 processes share 4 cores and
> > > 8 hw threads right? This is what we are seeing mostly
> >
> > I expect the 8 cgA tasks to spread on each core, instead of occupying
> > 4 cores/8 hw threads. If they stay on 4 cores/8 hw threads, than on the
> > core level, these cores' load would be much higher than other cores
> > which are running cgB's tasks, this doesn't look right to me.
> >
>
> I don't think that's a valid assumption, at least since the load balancer rework.
>
> The scheduler will be looking much more at the number of running task versus
> the group weight. So in this case 2 running tasks, 2 siblings at the core level
> will look fine. There will be no reason to migrate.

Can this be replicated?

>
> > I think the end result should be: each core has two tasks queued, one
> > cgA task and one cgB task(to maintain load balance on the core level).
> > The two tasks are queued on different hw thread, with cgA's task runs
> > most of the time on one thread and cgB's task being forced idle most
> > of the time on the other thread.
> >
>
> With the core scheduler that does not seem to be a desired outcome. I think
> grouping the 8 cgA tasks on the 8 cpus of 4 cores seems right.
>
Especially, if the load of cgA task + cgB task > cpu capacity, grouping cgA
tasks can avoid forced idle completely. Maintaining core level balance seems
not the best result. I guess that's why with core scheduler enabled we saw
10-20% improvement in some cases against the default core scheduler disabled.

Thanks,
-Aubrey