RFR: 8268906: gc/g1/mixedgc/TestOldGenCollectionUsage.java assumes that GCs take 1ms minimum

[Thomas Schatzl]

On Fri, 18 Jun 2021 18:00:06 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Hi,
>> 
>>   can I have reviews for this change to a test that incorrectly assumed that GCs take 1ms minimum?
>> 
>> At the end of the test, MXBeans' `getCollectionTime()` is compared against one taken two gcs earlier. If these two gcs are very short, `getCollectionTime()` returns the same value as before because its resolution is 1ms.
>> 
>> The `getCollectionTime()` documentation states:
>> 
>>> Returns the approximate accumulated collection elapsed time in milliseconds. This method returns -1 if the collection elapsed time is undefined for this collector.
>>> 
>>> The Java virtual machine implementation may use a high resolution timer to measure the elapsed time. **This method may return the same value even if the collection count has been incremented if the collection elapsed time is very short.**
>>> 
>> 
>> Which is what the test tested
>> 
>> Testing: test does not fail any more after some VM hack so that `getCollectionTime()` returns very low values.
>> 
>> Thanks,
>>   Thomas
>
> Looks good.

Thanks @kimbarrett for your review.

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PR: https://git.openjdk.java.net/jdk/pull/4531