Re: [PATCH AUTOTEST] kvm: timedrift test: fix typo (host_delta_t)

[Uri Lublin <uril@redhat.com>]

On 02/03/2010 04:47 PM, Michael Goldish wrote:
>
> ----- "Uri Lublin"<uril@redhat.com>  wrote:
>
>> Signed-off-by: Uri Lublin<uril@redhat.com>
>> ---
>>   client/tests/kvm/tests/timedrift.py |    2 +-
>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/client/tests/kvm/tests/timedrift.py
>> b/client/tests/kvm/tests/timedrift.py
>> index b3e8770..06f6a70 100644
>> --- a/client/tests/kvm/tests/timedrift.py
>> +++ b/client/tests/kvm/tests/timedrift.py
>> @@ -160,7 +160,7 @@ def run_timedrift(test, params, env):
>>       # Report results
>>       host_delta_total = ht2 - ht0
>>       guest_delta_total = gt2 - gt0
>> -    drift_total = 100.0 * (host_delta_total - guest_delta_total) /
>> host_delta
>> +    drift_total = 100.0 * (host_delta_total - guest_delta_total) /
>> host_delta_total
>
> This isn't a typo.
> delta_total is the load duration (e.g. 1 min of video decoding) +
> rest duration (e.g. 20 secs of idleness).
> I think the load drift and the total drift should be divided by the
> same delta, in order to determine the amount of drift corrected during
> idleness.  If you divide the total drift by delta_total (instead of
> delta) you give a false impression that more drift was corrected than
> really was.
>
> I'm not sure I'm making my point clearly so here's an example:
>
> Let's assume:
> - The load duration is 30s;
> - the idle duration is 30s;
> - the drift was 10s;
> - the drift was not corrected at all during idleness -- so after the
> idle period the drift remained 10s.
>
> Then:
> - The "load drift" is 33.3% (10/30);
> - if you divide by delta, the "total drift" is still 33.3% (10/30);
> - if you divide by delta_total, the "total drift" is 16.6% (10/60).
>
> So dividing by delta_total gives the impression that some drift was
> corrected, when in fact none was.
>

O.K. That makes sense. Thanks for the explanation.

In your example, the "total drift" is 33% of "the load duration", which is a bit 
confusing.