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

[Albert Mingkun Yang]

On Fri, 18 Jun 2021 17:43:05 GMT, Thomas Schatzl <tschatzl 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

Marked as reviewed by ayang (Committer).

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