Re: [Xen-devel] credit based scheduler

[Emmanuel Ackaouy <ack@xxxxxxxxxxxxx>]

On Wed, Jun 21, 2006 at 09:39:17PM +0200, Molle Bestefich wrote:
> Emmanuel Ackaouy wrote:
> Sounds like this algorithm is an evil killer of CPU cache performance to me 
> :-).

I don't agree.

First of all, if you were to time slice two VCPUs on one PCPU
and dedicate the other PCPU for the 3rd VCPU, you'd still be
time-slicing on one PCPU and you'd pay context switching and
cache sharing costs there.

Second, migration or not, once you start time-slicing, you have
to deal with cache warming costs. The important thing is to run
long enough time slices to take advantage of the cache.

Third, there are many applications where latency to run is more
important than wasting idle CPU cycles in order to run on a
warm cache.

Fourth, in the case we were talking about of 3 active VCPUs on
a 2PCPU host: Without migration, 2 VCPUs time-slice and get 25%
of the total host CPU resources each. 1 VCPU has 50% and does
not time-slice. With migration, all 3 VCPUs time-slice and get
33% of the total CPU resources. Two out of three VCPUs do better
with migration and you end up with fair sharing of system-wide
CPU resources too.

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