RFR: 8242440: use separate, destroyable JavaVM instances per libgraal compiler thread

[David Holmes]

On Fri, 15 Apr 2022 13:29:43 GMT, Doug Simon <dnsimon at openjdk.org> wrote:

> Currently, libgraal runs in a single, permanent JavaVM instance loaded from libjvmcicompiler.so. This poses 2 problems:
> 
> 1. The memory used for libgraal is never released, even when the libgraal compiler queues are idle.
> 2. Since all libgraal HotSpot compiler threads share the same SVM heap, libgraal compilation is effectively paused when a GC occurs (SVM currently has a stop the world collector). The more compiler threads there are, the more often this happens.
> 
> This PR implements the following solution to these problems:
> 1. Create a new JavaVM instance per libgraal compiler thread (i.e. a separate SVM isolate per thread). This prevents GC in one libgraal thread from impacting another libgraal thread.
> 2. Destroy the JavaVM instances when a libgraal compiler thread becomes idle and create a new one when subsequent compilation requests arrive.
> 
> Most of the changes are in JVMCI specific files. The most significant change to shared code is the addition of the `JavaThread:: _libjvmci_runtime` field. This is required as any thread (not just a `CompilerThread`) can be associated with a specific `JVMCIRuntime` object.

@dougxc I am not clear on what you are doing here, as we don't support loading the JVM more than once per-process and "destroying" a JVM doesn't return a lot of the resources you might expect to be returned.

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PR: https://git.openjdk.java.net/jdk/pull/8262