Re: AW: High xen_hypercall_sched_op usage

[Juergen Gross <jgross@xxxxxxxx>]

On 14.11.23 18:42, Klaus Darilion wrote:
> Hi Juergen!
>
> > -----Ursprüngliche Nachricht-----
> > Von: Juergen Gross <jgross@xxxxxxxx>
> > Gesendet: Dienstag, 14. November 2023 17:14
> > An: Klaus Darilion <klaus.darilion@xxxxxx>; xen-users@xxxxxxxxxxxxx
> > Betreff: Re: High xen_hypercall_sched_op usage
> >
> > > So most of CPU time is consumed by xen_hypercall_sched_op. IS it normal
> > that
> > > xen_hypercall_sched_op
> > >
> > > basically eats up all CPU? Is this an indication of some underlying problem?
> > Or
> > > is that normal?
> > In a PV guest the sched_op hypercall is used e.g. for going to idle. I guess
> > you are adding up all idle time to the sched_op hypercall.
> What does this mean? Idle Time is measured as "system cpu usage"? Further, the 
> VM was getting real slow while that amount increase, so I think the VM was not idle but 
> waiting for something.
Depends how your numbers are being collected. If the collection process is
using a statistical approach (looking which function is interrupted by timer
interrupts), the idle function is a natural candidate to show up rather
prominent.

Other sched_op use cases are e.g. the "poll" call, which is used when waiting
for a spinlock to become free. lockstat will be your friend in case this is
the reason for your high percentage of the sched_op function.

> > > b1) So, as the VMs are not pinned, it may happen that the same CPU is
> > used for
> > > the dom0 and the domU. But why? There are 128vCPUs available, and only
> > 112vCPUs
> > > used. Is XEN not smart enough to use all vCPUs?
> > You are mixing up vcpus and physical cpus.
> >
> > A vcpu is a virtualized cpu presented to the guest. It can run on any physical
> > cpu if no pinning etc. is involved.
> Will it be beneficial if all VMs have hard pinned CPUs?
This can't be told without knowing the exact workload. And "exact" means
down to cache usage, timing, ...

You can try, of course.

As soon as you have more vcpus (sum over all domains) than physical cpus
chances are good that pinning will make things worse.


Juergen

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