RFR: 8283276: java/io/ObjectStreamClass/ObjectStreamClassCaching.java fails with various GCs

[Peter Levart]

On Mon, 18 Jul 2022 07:40:53 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Test appears to pass fine with G1. But it fails with other GCs, for example Parallel, Shenandoah, etc, it fails:
> 
> 
> $ CONF=linux-x86_64-server-fastdebug make test TEST=java/io/ObjectStreamClass/ObjectStreamClassCaching.java TEST_VM_OPTS="-XX:+UseParallelGC"
> 
> test ObjectStreamClassCaching.testCacheReleaseUnderMemoryPressure(): success
> test ObjectStreamClassCaching.testCachingEffectiveness(): failure
> java.lang.AssertionError: Cache lost entry although memory was not under pressure expected [false] but found [true]
> 	at org.testng.Assert.fail(Assert.java:99)
> 	at org.testng.Assert.failNotEquals(Assert.java:1037)
> 	at org.testng.Assert.assertFalse(Assert.java:67)
> 
> 
> I believe this is because `System.gc()` is not that reliable about what happens with weak references. As seen with other GCs, they can clear the weakrefs on Full GC. In fact, the test fails with G1 if we do a second System.gc() in this test. So the test itself is flaky. The fix is to avoid doing `System.gc()` altogether in that subtest. The test is still retained to see that reference is not cleared for a while.
> 
> Additional testing:
>  - [x] Linux x86_64 fastdebug, affected test with `-XX:+UseSerialGC`, 100 repetitions
>  - [x] Linux x86_64 fastdebug, affected test with `-XX:+UseParallelGC`, 100 repetitions
>  - [x] Linux x86_64 fastdebug, affected test with `-XX:+UseG1GC`, 100 repetitions
>  - [x] Linux x86_64 fastdebug, affected test with `-XX:+UseShenandoahGC`, 100 repetitions
>  - [x] Linux x86_64 fastdebug, affected test with `-XX:+UseZGC`, 100 repetitions

Just a note that the failing test is not about checking that cache is cleared after plain System.gc(), but about checking that cache is NOT cleared after System.gc() when there was no real memory pressure. The cache uses SoftReference(s). So it appears that some gc(s) do clear SoftReference(s) on System.gc(). Does WhiteBox GC util have a means to provoke gc without SoftReference processing?

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PR: https://git.openjdk.org/jdk/pull/9533