Re: [PATCH] kvm-vmx: add module parameter to avoid trapping HLT instructions (v2)

[Chris Wright <chrisw@sous-sol.org>]

* Anthony Liguori (anthony@codemonkey.ws) wrote:
> On 12/02/2010 08:42 PM, Chris Wright wrote:
> >OK, let's say a single PCPU == 12 Compute Units.
> >
> >If the guest is the first to migrate to a newly added unused host, and
> >we are using either non-trapping hlt or Marcelo's non-yielding trapping
> >hlt, then that guest is going to get more CPU than it expected unless
> >there is some throttling mechanism.  Specifically, it will get 12CU
> >instead of 1-3CU.
> >
> >Do you agree with that?
> 
> Yes.
> 
> There's definitely a use-case to have a hard cap.

OK, good, just wanted to be clear.  Because this started as a discussion
of hard caps, and it began to sound as if you were no longer advocating
for them.

> But I think another common use-case is really just performance
> isolation.  If over the course of a day, you go from 12CU, to 6CU,
> to 4CU, that might not be that bad of a thing.

I guess it depends on your SLA.  We don't have to do anything to give
varying CU based on host load.  That's the one thing CFS will do for
us quite well ;)

> If the environment is designed correctly, of N nodes, N-1 will
> always be at capacity so it's really just a single node hat is under
> utilized.

Many clouds do a variation on Small, Medium, Large sizing.  So depending
on the scheduler (best fit, rr...) even the notion of at capacity may
change from node to node and during the time of day.

thanks,
-chris