Bryan Says...

Rants, opinions, and other stuff I find interesting.

Tune your Ruby Enterprise Edition garbage collection settings to run tests faster

From what I understand, not enough people know about the tunable garbage collector (GC) provided by Ruby Enterprise Edition.  Section 4.2 of the REE documentation gives you the run-down.

Now that you read the documentation, why aren’t you tuning your GC?  This is one of the easiest ways to speed up your test runs.  Here is a quick example of the gains that can be achieved just by setting 5 environment variables.


With out tuning

410 scenarios (410 passed)
3213 steps (3213 passed)

With tuning

410 scenarios (410 passed)
3213 steps (3213 passed)

Keep in mind, all I did was change 5 settings.

export RUBY_HEAP_MIN_SLOTS=1000000
export RUBY_GC_MALLOC_LIMIT=1000000000
export RUBY_HEAP_FREE_MIN=500000

How do these settings effect your application during test runs, and how can you make your application (running on REE) faster?  Well, let’s go over what we have. I will annotate the existing documentation with the reasoning for my settings:


This specifies the initial number of heap slots. The default is 10000.

The minimum amount of heap slots is pretty small.  Since this configuration is just for my tests, I’m sure I will use way more slots, so I’m going to start of with more.  This means that initially my test process will consume more memory.


The number of additional heap slots to allocate when Ruby needs to allocate new heap slots for the first time. The default is 10000.

For example, suppose that the default GC settings are in effect, and 10000 Ruby objects exist on the heap (= 10000 used heap slots). When the program creates another object, Ruby will allocate a new heap with 10000 heap slots in it. There are now 20000 heap slots in total, of which 10001 are used and 9999 are unused.


From what I understand, when the slots are all consumed, a new batch of slots will be allocated.  Once again, this number is way too low, so I increase it by a factor of 100.


Multiplicator used for calculating the number of new heaps slots to allocate next time Ruby needs new heap slots. The default is 1.8.

Take the program in the last example. Suppose that the program creates 10000 more objects. Upon creating the 10000th object, Ruby needs to allocate another heap. This heap will have 10000 * 1.8 = 18000 heap slots. There are now 20000 + 18000 = 38000 heap slots in total, of which 20001 are used and 17999 are unused.

The next time Ruby needs to allocate a new heap, that heap will have 18000 * 1.8 = 32400 heap slots.


Since my increment is so large, I’m only going to use a factor of 1.

The amount of C data structures which can be allocated without triggering a garbage collection. If this is set too low, then the garbage collector will be started even if there are empty heap slots available. The default value is 8000000.

This is my development box, so feel free to use as much memory as you want.  I’m sure there is an upper limit on this number, but I haven’t hit it yet.

The number of heap slots that should be available after a garbage collector run. If fewer heap slots are available, then Ruby will allocate a new heap according to the RUBY_HEAP_SLOTS_INCREMENTand RUBY_HEAP_SLOTS_GROWTH_FACTOR parameters. The default value is 4096.

Since we have lots of memory, we’ll just make this half of our minimum number of slots.

Please do not use these settings in production. Both 37signals and Twitter have provided their settings, and you can use those as a basis for tuning your app.

Thanks to Evan Weaver and Nick Gauthier for most of the guts of this blog post.

  • Anonymous

    Can you please post you configuration (Ram, cpu etc)? I can’t get any improvements even with smaller values on my machine – Ubuntu server 10.04 – 1gb of ram but in VirtualBox.

  • bryanl

    I’ve tested this on a quad core i7 iMac and a an i7 macbook pro. I saw 30%+ gains on both. Both boxes have 8GB of memory. I haven’t tested these particular settings on anything smaller. I’m going to fire up parallels right now and give it a try.

  • bryanl

    I tried it on an Ubuntu 10.10 VM with 1GB of ram and 1CPU. 42% gain. How long do your tests run?

  • Lawrence Pit

    I’m not in a position to test this myself atm (my dev compu is getting a new motherboard), just wondering what would happen if you disable GC altogether: GC.disable ?

  • technicalpickles

    Just tried this out on our suite that takes 27s, and it went down to about 18s. 33% improvement, not bad :)

    Are you setting these in your shell environment so they apply everywhere, or just choosing to do it more selectively when it’s test-time?

  • Anonymous

    12m54.595s with default settings vs 13m0.154s with your values. At one point it just starts swapping so that is probably the problem… disk io kills the joy :) I will try with 2gb of ram.

  • nevans

    In case you are only just learning about these environment variables, if you aren’t tweaking them in production, you *need* to. I cut my average request time almost in half. Just don’t use the same settings as Bryan gives for testing on a dev box. Use something more similar to what 37signals and Twitter use (Bryan linked to gists, and it’s also in the REE documentation). Thanks Bryan!

  • Nick Gauthier

    Keep a close watch on your memory usage. Like a server, the minute you start swapping performance goes out the window.

  • Alex Sharp

    Nice post Bryan. We’ve been taking advantage of these settings for about a month and some people have experienced > 50% improvements. Another important factor I’ve noticed if you’re using MySQL are innodb-specific settings you set in the mysql config. I have this in my /etc/my.cnf file:

    Thanks again!

  • Alex Sharp

    Absolutely, this kills me all the time.

  • Bradley Grzesiak

    We have a wrapper script around rake called “cr”, which:
    * clears the screen
    * sets these REE GC settings
    * times the execution of rake (see time(1))
    * reports via the OS X “say” command the success or failure of the rake command

    We do this within “cr” rather than the terminal environment because running rails console or (especially) server on a dev box with these values might get you a bit too close to swapping.

    Also, it’s nice to have an audio cue as to when your tests are finished if they take more than 10 seconds. You can go off and do other things and be notified when rake finishes.

  • Millisami

    But you didn’t tell it on how to use it? Where should I put those configurations?? in some file inside my test env or elsewhere??

  • Karle

    Millisami brings up a good point, what’s the best way to go about setting these variables for just your tests? Using a wrapper script to run rake test?

  • Millisami

    Can you or anyone in this comment thread explain where/in which file/ to put these settings?
    All are showing their performance improvements percentages and seconds, but no one willing to open this up for others!!
    Shame on me or u guys!!!

  • bryanl
  • bryanl
  • Kevin Carter

    dude how old u is