Re: [Qemu-devel] [PATCH v5 1/5] cpu: Provide vcpu throttling interface

["Jason J. Herne" <jjherne@linux.vnet.ibm.com>]

On 09/01/2015 05:23 PM, Eric Blake wrote:
> On 09/01/2015 08:45 AM, Jason J. Herne wrote:
>> Provide a method to throttle guest cpu execution. CPUState is augmented with
>> timeout controls and throttle start/stop functions. To throttle the guest cpu
>> the caller simply has to call the throttle set function and provide a percentage
>> of throttle time.
>>
>> Signed-off-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
>> Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
>> ---
>>   cpus.c            | 78 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>   include/qom/cpu.h | 43 ++++++++++++++++++++++++++++++
>>   2 files changed, 121 insertions(+)
>>
>
>> +/**
>> + * cpu_throttle_set:
>> + * @new_throttle_pct: Percent of sleep time to running time.
>
> I don't think you mean the ratio of sleep time to running time, as much
> as sleep time to overall time.
>
> That is, 10% should mean "sleep 1ms and run 9ms out of every 10ms
> total", and not "sleep 1ms after every 10ms run, for 90.9% duty cycle".
>
>> + *                    Valid range is 1 to 99.
>> + *
>> + * Throttles all vcpus by forcing them to sleep for the given percentage of
>> + * time. A throttle_percentage of 50 corresponds to a 50% duty cycle roughly.
>> + * (example: 10ms sleep for every 10ms awake).
>
> 50% can be a bit ambiguous (not obvious whether a higher percentage
> means more sleep or more time awake); it might be better to pick a
> different number, such as:
>
> A throttle_percentage of 25 corresponds to a 75% duty cycle (example:
> 10ms sleep for every 30ms awake).
>
> if the percentage really is [1 - duty cycle] (which I suspect), or:
>
> A throttle_percentage of 25 corresponds to a 80% duty cycle (example:
> 10ms sleep for every 40ms awake).
>
> (if I'm wrong, and you really meant percentage of sleep time to running
> time).
>

You are 100% correct here. This comment is a relic of an older version. 
Nice find :)
I'll update the docs and incorporate your suggestions.

-- 
-- Jason J. Herne (jjherne@linux.vnet.ibm.com)