Switch docs to recommend mini_racer over execjs

[justin808]

https://github.com/discourse/mini_racer might have a big speed up.

Let's try it out!

Any volunteers?

Performance

The bench folder contains benchmark.

Benchmark minification of Discourse application.js (both minified and unminified)

• MiniRacer version 0.1
• therubyracer version 0.12.2

$ ruby bench_uglify.rb
Benching with MiniRacer
MiniRacer minify discourse_app.js 13813.36ms
MiniRacer minify discourse_app_minified.js 18271.19ms
MiniRacer minify discourse_app.js twice (2 threads) 13587.21ms

Benching with therubyracer
MiniRacer minify discourse_app.js 151467.164ms
MiniRacer minify discourse_app_minified.js 158172.097ms
MiniRacer minify discourse_app.js twice (2 threads) - DOES NOT FINISH

Killed: 9

The huge performance disparity (MiniRacer is 10x faster) is due to MiniRacer running latest version of V8. In July 2016 there is a queued upgrade to therubyracer which should bring some of the perf inline.

Note how the global interpreter lock release leads to 2 threads doing the same work taking the same wall time as 1 thread.

[jbhatab]

That's a big improvement.

[justin808]

@jbhatab Any chance that you can try this out? Maybe we can put this into v.7?

[justin808]

@robwise Please post some notes here when you get a chance. @robwise told me that MiniRacer was much faster. We really need a PR with doc updates on this.