Performance is bad when running mini_racer tests

[lloeki]

Doesn't seem to affect Linux (glibc at least). Has happened on musl at least once.

Example on macOS (libv8-node):

Fabulous run in 90.207620s, 0.9201 runs/s, 1.5409 assertions/s.

vs (libv8)

Fabulous run in 2.528212s, 32.8295 runs/s, 54.9796 assertions/s.

[lloeki]

We pinpointed the performance thing to MiniRacer::Context.new:

bundle exec ruby -e 'require "benchmark"; Benchmark.bm { |b| context = nil; b.report { require "mini_racer" }; b.report { context = MiniRacer::Context.new }; b.report { 100.times { context.eval("function f(x) { return \"I need  \"+ x + \" foos\" }") } } }'

gives (libv8-node)

       user     system      total        real
   0.012711   0.005495   0.018206 (  0.662711)
   0.815646   0.021527   0.837173 (  0.837263)
   0.003013   0.002043   0.005056 (  0.003985)

vs (libv8)

       user     system      total        real
   0.011028   0.003825   0.014853 (  0.014875)
   0.002350   0.000868   0.003218 (  0.002852)
   0.003491   0.001899   0.005390 (  0.004307)

Basically all that time is spent when calling MiniRacer::Context.new, to which the test suite is sensitive to, but otherwise it's fine.

More specifically it's Isolate::New. From some lousy printf profiling of rb_context_init_unsafe then IsolateInfo::init:

init took 602 us
new isolateinfo took 32 us
isolateinfo init
    allocator took 2 us
    params took 0 us
    Isolate::New took 868582 us
  took 868605 us
isolateinfo hold took 0 us
isolateinfo lock took 1431 us
datetime took 13 us

[lloeki]

• Built with small-icu
• Rearranged lib building