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Rails Tests Run in 2/3 Time w/ GC Tuning

12/10/2010

Run Your Unit Tests in 2/3 the Time
Tweaking the Ruby Enterprise Edition (REE) garbage collection (GC) parameters, I was able to run my unit tests in 2/3 the normal time. Total test time w/ Ruby 1.8.7 down from 20mins to approx 6mins on tuned REE 1.8.7.

This data was measured on the PatientsLikeMe Rails codebase, a very mature and large Rails app. The hardware is a MacBook Pro w/ Rails 2.3.5 on OSX 10.6.4. Your mileage may vary.

Background: Garbage Collection & Tuning
Ruby is a dynamic language with GC managing dynamic memory allocation. Most Ruby programmers have the benefit of ignoring the garbage collector during development, but tuning the GC parameters can have dramatic benefits in production and running your tests locally. Using REE allows the tuning of many GC parameters.

37Signals Production Settings

# NOTE: These only take effect when running Ruby Enterprise Edition

export RUBY_HEAP_MIN_SLOTS=600000
export RUBY_GC_MALLOC_LIMIT=59000000
export RUBY_HEAP_FREE_MIN=100000

Measured Performance

# Before (REE, no GC settings)
#
$> ruby -v
ruby 1.8.7 (2010-04-19 patchlevel 253) [i686-darwin10.4.0], MBARI 0x6770, Ruby Enterprise Edition 2010.02
$> rake test:units
#Snip...
Finished in 666.310269 seconds.

3883 tests, 11523 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
# After (REE, w/ 37Signals GC tuning)
#
$> ruby -v
ruby 1.8.7 (2010-04-19 patchlevel 253) [i686-darwin10.4.0], MBARI 0x6770, Ruby Enterprise Edition 2010.02
$> env | grep RUBY
RUBY_HEAP_FREE_MIN=100000
RUBY_HEAP_MIN_SLOTS=600000
RUBY_GC_MALLOC_LIMIT=59000000
$> rake test:units
#Snip...
Finished in 411.319884 seconds.

3883 tests, 11523 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications

Why? What Do These Settings Mean?

  • RUBY_HEAP_MIN_SLOTS – Number of slots in Ruby heap, directly controls initial heap size in your VM. Should be large enough to hold entire Rails environment. This is 6x the default heap size.
  • RUBY_GC_MALLOC_LIMIT – Wait until # of malloc() calls to trigger GC, this is much longer wait than Ruby default period for less collections. The value above collects every 59mil malloc()s
  • RUBY_HEAP_FREE_MIN – Minimum free heap post-collection, if not met will allocate a whole new heap. We’ve set it here to 17% the size of the heap. Default is 25% of heap.
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