RFR: 8242440: use separate, destroyable JavaVM instances per libgraal compiler thread [v3]

[Doug Simon]

On Wed, 20 Apr 2022 20:56:19 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

> Preliminary comment. I agree with David that calling additional library `JavaVM` is confusing and is not correct. This is SVM: "The former each encapsulate a single SubstrateVM runtime". May be you should called as such in code and comments.

`JavaVM` is a JNI defined type and HotSpot instantiates and communicates with libgraal purely through JNI so I believe it is the right terminology. The fact that libgraal is built with SVM is an implementation detail that HotSpot is not aware of. That's why "SVM" does not appear anywhere in the JVMCI C++ code.

> src/hotspot/share/jvmci/jvmci_globals.hpp line 64:
> 
>> 62:           "Max number of threads per JVMCI native runtime. "                \
>> 63:           "Specify 0 to force use of a single JVMCI native runtime. ")      \
>> 64:           range(0, max_jint)                                                \
> 
> Does it mean that value `0` has the same effect as `1`?

No. `0` means there is a single JVMCI native runtime shared by all threads where as `1` means each thread (that accesses JVMCI) gets its own unique isolate.

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