Set JTREG concurrency based on cores and memory size

[jerboaa]

Upstream OpenJDK test framework, roughly, sets it to
min(NPROCS/2, MEM_IN_GB/2). Set it to a similar value
so as to avoid test reliability issues due to memory
limits.

[jerboaa]

Grinder on the big Aarch64 box with this:
https://ci.adoptopenjdk.net/job/Grinder_Sandbox/884/console

It's rather conservative but certainly better than a concurrency value of NPROCS. I'll do some more testing on other boxes and may amend it. Hence, the WIP suffix.

[jerboaa]

@smlambert Thoughts?

[smlambert]

Thanks @jerboaa - have been meaning to look at what we were currently doing, based on the troubles seen running on the 'big box' / test-packet-armv8-ubuntu-16-04 reported in #1203 and seen in a bunch of the recent release runs (seen more frequently now that there are less aarch64 machines in the pool.

Agree its an improvement, will be good to see it run through the other platforms to make sure it doesn't hose anything, thanks for looking at this !

[jerboaa]

Windows x86_64, JDK 8:
https://ci.adoptopenjdk.net/job/Grinder_Sandbox/886/

Linux ppcle, JDK 13:
https://ci.adoptopenjdk.net/job/Grinder_Sandbox/887/

Mac x86_64, JDK 8:
https://ci.adoptopenjdk.net/job/Grinder_Sandbox/888/

[jerboaa]

Another run on the 96 core aarch64 box (on upstream aarch64 JDK 11.0.6+1 EA build):
https://ci.adoptopenjdk.net/job/Grinder_Sandbox/889/

Contrast this to flaky runs earlier on the same JDK: 11.0.6+1 EA. For example:
https://ci.adoptopenjdk.net/view/Test_upstream/job/Test_openjdk11_hs_sanity.openjdk_aarch64_linux_upstream/23/consoleFull

[jerboaa]

This seems to build OK on Windows/Linux/Mac. I'm removing the WIP prefix.

[jerboaa]

@smlambert More testing of this completed. Make file seems OK. I don't think any of the failures are related to this patch. Mac/Win (no JRE 8 got downloaded, hence failure). Aarch64 linux on the big box now passes (or is significantly better than before in terms of reliability). ppcle 64 bit linux failures for JDK 13 also seem unrelated.

I'd appreciate somebody reviewing this! Thanks!

[jerboaa]

Thanks, @smlambert!