Tuning JVM options for throughput

[DavidGregory084]

Changes the JVM options used by SBT somewhat.

Here is the reasoning for each change:

• -Xmx4G - the Travis infrastructure that we're using has 7.5GB of memory, this could probably be increased further if forking tests was also disabled
• removal of XX:-UseGCOverheadLimit - I think that this is just creating zombie builds that are stuck thrashing the GC and will timeout anyway due to lack of progress, better to fail early IMO
• removal of -XX:+CMSClassUnloadingEnabled - this is relevant only for the CMS collector
• switch from -XX:+UseConcMarkSweepGC to -XX:+UseParallelGC - from what I understand the concurrent CMS collector is a response time collector, whereas the parallel GC is a throughput collector (more on this in the Oracle docs), which is much more relevant for compiling code IMO

Here is some VisualVM output from running sbt clean test/clean catsJS/test, which normally stalls for me locally with the default options.

Before:

Note that I had to kill this build as it got stuck.

After:

Hopefully this will translate to improvements on Travis too! Who knows!

[DavidGregory084]

One more comment about the choice of GC; the Scala compiler team noted performance regressions when moving to Java 9 due to the default GC changing from the ParallelGC (a throughput collector) to the GC1C (a response-time collector).

[codecov-io]

Codecov Report

Merging #2797 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2797      +/-   ##
==========================================
- Coverage   94.28%   94.26%   -0.02%     
==========================================
  Files         367      367              
  Lines        6926     6927       +1     
  Branches      297      297              
==========================================
  Hits         6530     6530              
- Misses        396      397       +1

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[DavidGregory084]

EDIT: I'm looking at the wrong times in travis...

The first build took ~30min, which is consistent with other recent successful builds

Going to close and reopen to give it another go and see how consistent it is

[DavidGregory084]

Second build took ~31min

On that basis I'm not sure this is going to yield any improvement in build times, but it may be less likely that builds will freeze on account of memory usage

[kailuowang]

Thanks so much! @DavidGregory084, I am not an expert on concurrent CMS collector v.s. parallel GC, but I vote that we give it a try.

[tpolecat]

Cool, I need to try these options too!