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

[Avi Kivity <avi@redhat.com>]

On 12/02/2010 05:23 PM, Anthony Liguori wrote:
> On 12/02/2010 08:39 AM, lidong chen wrote:
>> In certain use-cases, we want to allocate guests fixed time slices 
>> where idle
>> guest cycles leave the machine idling.
>>
>> i could not understand why need this? can you tell more detailedly?
>
> If you run 4 guests on a CPU, and they're all trying to consume 100% 
> CPU, all things being equal, you'll get ~25% CPU for each guest.
>
> However, if one guest is idle, you'll get something like 1% 32% 33% 
> 32%.  This characteristic is usually desirable because it increase 
> aggregate throughput but in some circumstances, determinism is more 
> desirable than aggregate throughput.
>
> This patch essentially makes guest execution non-work conserving by 
> making it appear to the scheduler that each guest wants 100% CPU even 
> though they may be idling.
>
> That means that regardless of what each guest is doing, if you have 
> four guests on one CPU, each will get ~25% CPU[1].
>

What if one of the guest crashes qemu or invokes a powerdown?  Suddenly 
the others get 33% each (with 1% going to my secret round-up account).  
Doesn't seem like a reliable way to limit cpu.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.