RFR (xxs) JDK-8012525 gc/metaspace/G1AddMetaspaceDependency.java Test fails a safepoint timeout assertion or hangs.

[Mikael Gerdin]

Hi

Please review this small test fix to work around a problem where the 
test accidentally provokes a very slow execution path in the VM.
This slow path can cause very long GC pauses or delayed safepoints.

Summary of the bug:
The byte array allocation causes a big MemRegion to be stored as the 
thread's deferred_card_mark. This can cause two separate slow code paths:

* A slow GC pause because CollectedHeap::ensure_parsability flushes the 
cards through the VM barrier code and dirtying a large region is 
expensive with the small G1UpdateBufferSize.
* A thread_in_java being slow in reaching safepoint because it's 
flushing the cards in OptoRuntime::new_array_C at the call to 
new_store_pre_barrier, this ends up in the same code as 
ensure_parsability and can therefore be slow.

I've ran the test with the large allocation removed and it still 
successfully reproduces the bug it was written to provoke on an unfixed 
build, so my suggestion is to simply remove the byte array allocation.

The bug is unfortunately not externally visible.

Webrev: http://cr.openjdk.java.net/~mgerdin/8012525/webrev

/Mikael