RE: [Xen-devel] Power aware credit scheduler

["Ian Pratt" <Ian.Pratt@xxxxxxxxxxxxx>]

> c) when cpu's freq is scaled dynamically
>       When cpufreq/Px is enabled, cpu's frequency is adjusted
> to different operation points driven by a on-demand governor. So
> csched_acct may need take frequency difference among cpus into
> consideration and total available credits won't be a simple 300 *
> online cpu_number.

We should also adjust the accounting of the credits consumed in light of
hyperthreading: we should scale the credit we subtract proportional to
the how much of the period was spent competing with another VCPU running
on alternate hyperthread (we can tell this by seeing how much time the
idle thread spent running on the other thread). 

We can then scale the accounting according to some rough notion of the
expected throughput of two hyperthreads e.g. experience on P4 CPU's
suggests that a single VCPU will typically receive something like 65% of
its normal throughput when competing against another thread (total
throughput 130%). We thus scale the amount of credit subtracted between
65% and 100% depending on how much time was spent competing. 

There's an argument that says we should at least have an option to
prevent VCPUs from different guests running against each other in
adjacent threads. This would be introducing a simple kind of gang
scheduling.

Ian  

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