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

I managed to construct/fix test that passes on G1, Shenandoah and ZGC...

https://github.com/plevart/jdk/commit/2dbce34d678bee995c808f3bb2a021bcc98b5d01

The combination of JVM options: -Xmx10m -XX:SoftRefLRUPolicyMSPerMB=1  ... makes ZGC and Shenandoah very aggressive and practically causes SoftReferences to be treated equally as WeakReferences. By increasing the max. heap size a bit and using defaults for SoftRefLRUPolicyMSPerMB (I believe 1000 ms/MB for G1 and 200 ms/MB for Shenandoah and ZGC), G1 and Shenandoah were happy with the changed test. For ZGC I also had to make sure that the CacheEffectiveness test is executed as 1st test in VM instance, followed by CacheReleaseUnderMemoryPressure.

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