Re: [Xen-devel] Notes on stubdoms and latency on ARM

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 7 Jul 2017, Volodymyr Babchuk wrote:
> >> I run test in DomU:
> >> real 113.08
> >> user 0.00
> >> sys 113.04
> >>
> > Ok, so there's contention for pCPUs. Dom0's vCPUs are CPU hogs, while,
> > if my assumption above is correct, the "SMC vCPU" of the DomU is I/O
> > bound, in the sense that it blocks on an operation --which turns out to
> > be SMC call to MiniOS-- then resumes and block again almost
> > immediately.
> >
> > Since you are using Credit, can you try to disable context switch rate
> > limiting? Something like:
> >
> > # xl sched-credit -s -r 0
> >
> > should work.
> Yep. You are right. In the environment described above (Case 2) I now
> get much better results:
> 
>  real 1.85
> user 0.00
> sys 1.85

From 113 to 1.85 -- WOW!

Obviously I am no scheduler expert, but shouldn't we advertise a bit
better a scheduler configuration option that makes things _one hundred
times faster_ ?! It's not even mentioned in
https://wiki.xen.org/wiki/Tuning_Xen_for_Performance!

Also, it is worrying to me that there are cases were, unless the user
tweaks the configuration, she is going to get 100x worse performance out
of her system.

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