Ruby 2.1: Method Cache

For years, MRI cleared the entire VM’s method cache whenever a new method was defined. In fact, method defintions were only one of a dozen ways to clear the method cache. Earlier this year, @jamesgolick decided to improve things with a patchset for 1.9 implementing hierarchical invalidation. @charliesome subsequently ported and committed the patchset to trunk. Starting with Ruby 2.1, altering a class will only invalidate the caches for that class and its subclasses.

To provide visibility, we’ve exposed some basic stats about the method cache via a new RubyVM.stat() method. For instance, an application can measure “global invalidations per request” by comparing RubyVM.stat(:global_method_state) before and after every request. See simeonwillbanks/busted for some convenience methods around these new stats.

To track down where (global and non-global) invalidations are happening, Ruby 2.1 ships with a new probe: ruby::method-cache-clear. This can easily be used via dtrace or systemtap to find the source of method cache invalidations in your application.

$ cat ruby_mcache.stp
probe process("/usr/bin/ruby").mark("method__cache__clear") {
    printf("%s(%d) %s %s:%d cleared `%s'\n", execname(), pid(), $$name, kernel_sring($arg2), $arg3, kernel_string($arg1))
}

$ sudo stap ruby_mcache.stp
ruby(25410) method__cache__clear lib/ruby/2.1.0/ostruct.rb:169 cleared `OpenStruct'
ruby(25410) method__cache__clear lib/ruby/2.1.0/ostruct.rb:170 cleared `OpenStruct'

Work on improving ruby’s method cache is continuing on ruby-core. Early numbers show up to 5-10% improvements are possible with a larger and more resilient cache. We hope to land some improvements to ruby-trunk (for 2.2), and maybe backport them into a future 2.1 patchlevel release.