Re: [RFC] Scheduler work, part 1: High-level goals and interface.

[Jeremy Fitzhardinge <jeremy@goop.org>]

Ian Pratt wrote:
>> I don't know what the performance characteristics of modern-HT is, but
>> in P4-HT the throughput of a given thread was very dependent on what the
>> other thread was doing. If its competing with some other arbitrary
>> domain, then its hard to make any estimates about what the throughput
>> of a given vcpu's thread is.
>>     
>
> The original Northwood P4's were fairly horrible as regards performance predictability, but things got considerably better with later steppings. Nehalem has some interesting features that ought to make it better yet.
>
> Presenting sibling pairs to guests is probably preferable (it avoids any worries about side channel crypto attacks), but I certainly wouldn't restrict it to just that: server hosted desktop workloads often involve large numbers of single VCPU guests, and you want every logical processor available.
>
> Scaling the accounting if two threads share a core is a good way of ensuring things tend toward longer term fairness.
>
> Possibly having two modes of operation would be good thing:
>
>  1. explicitly present HT to guests and gang schedule threads
>
>  2. normal free-for-all with HT aware accounting.
>
> Of course, #1 isn't optimal if guests may migrate between HT and non-HT systems.
>   

This can probably be extended to Intel's hyper-dynamic flux mode (that 
may not be the real marketing name), where it can overclock one core if 
the other is idle.

    J