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Ever read a web page about how to set your Ruby memory environment variables and thought, "but how do I know that's right for my app?" EnvMem is here to help you out.

Specifically, if you have a long-running or high-memory Ruby process (server, batch, etc) then your process will do more garbage collecting than is necessary in getting up to its long-term size. You can save a bit of time and processor by setting its environment variables close to their steady-state or end-of-process values.

This is the same thing you do when you set Ruby environment variables to more Rails-friendly, batch-friendly or your-server-friendly values from a web page. It's just that this way you can make sure it's a good match for your app, specifically.

EnvMem generates a small, simple shellscript to set your environment variable values. To use it, just source the script before running your application. You can manually tweak it later if you like, or remove variables you don't want to set for some reason - such as OLDMALLOC values if you've compiled a Ruby without it, for instance.


Add this line to your application's Gemfile:

gem 'env_mem'

And then execute:

$ bundle

Or install it yourself as:

$ gem install env_mem


EnvMem needs a dump of GC.stat values from your application in the configuration you want to match. If you have a long-running Rails server, that means after it has processed a bunch of HTTP requests. If you're using EnvMem to configure your batch script, that probably means dumping GC.stat after you've finished your batch work and your job's memory configuration is nice and stable.

Since you'll need the GC.stat values from the process, you'll need to dump them. First, here's how to do it without EnvMem:"gc_stat_dump.txt", "w") { |f| f.write GC.stat.inspect }

You can use EnvMem itself to dump GC.stat, but then you're using it at runtime. Here's how:

require 'env_mem'

To create the environment script from the stat dump, translate from one filename to another:

$ env_mem gc_stat_dump.txt >

Keep in mind that your application may change over time, and so it may need different memory settings. A simple way to handle that is to run your app without any Ruby memory environment variables set and then dump GC.stat again and regenerate them.

Long-Running Servers and Other Challenges

There isn't always an obvious way to get statistics at the start and the end of the process. Start is usually easy, but end can be a challenge. Here's something I've tried with a large Rails server that has worked okay:"gc_stats_#{}_start.txt", "w") { |f| f.print GC.stats.inspect }
at_exit {"gc_stats_#{}_stop.txt", "w") { |f| f.print GC.stats.inspect }

The "at_exit" block is saying that before the process exits, it should stop and write out the GC stats again. Doing this during teardown means you won't necessarily have an accurate count of how many active objects are currently sitting around... But most of your statistics will work great.

You can get a tiny bit of extra accuracy by instead adding a controller action to dump the GC stats while the Rails server is still fully active. But for most purposes, this will do just fine.

What the Variables Mean

Ruby has two obvious thresholds, "malloc" and "oldmalloc", that keep going up. The "malloc" limit is so that Ruby garbage-collects regularly every so many bytes allocated. The "oldmalloc" limit is to garbage collect as (its estimate of) the old-generation size in bytes increases.

Ordinarily a Ruby process will increase in size asymptotically, approaching its "full size." This is common for things like server processes that add and retain long-term memory (e.g. classes, caches) while adding a much smaller amount of per-request memory that gets garbage collected soon after the request is finished.

After each time the limit causes a major garbage collection (e.g. the total allocated size crosses the "malloc" limit), that limit is raised by a configurable "growth factor". For instance, with the default RUBY_GC_MALLOC_LIMIT_GROWTH_FACTOR of 1.4, the malloc limit will get 40% bigger each time. With a growth factor of 1.6, it would get 60% bigger. There can also be a LIMIT_MAX variable, so that the limit grows by the smaller of the growth factor or the limit max. For instance, with a growth factor of 1.6 and a limit max of 100,000, Ruby would grow its malloc limit by 60% each time until 60% was bigger than 100,000, and then it would grow by 100,000 each time.

The malloc (but not oldmalloc) limit will also slowly decrease back toward what you specify, by around 2% with each garbage collection. But not below the specified limit. The 2% is a "magic number" and not configurable.

Slots are slightly different than the malloc and oldmalloc limits - slots are fully managed by Ruby itself, while Ruby uses a system allocator to managed the malloc and oldmalloc systems.

With slots, Ruby starts with RUBY_GC_HEAP_INIT_SLOTS of them allocated. Slots also have a growth factor (RUBY_GC_HEAP_GROWTH_FACTOR) and a maximum growth (RUBY_GC_HEAP_GROWTH_MAX_SLOTS). But Ruby will only use them if you don't set ratios of free slots (see below.) By default, Ruby will aim for 40% of slots free, allocating more to reach this ratio. By default it will free pages of slots when at least 65% of its slots are free.

Here is a list of the variables in question:

  • RUBY_GC_HEAP_INIT_SLOTS - initial number of slots

  • RUBY_GC_HEAP_FREE_SLOTS - minimum free slots allowable after GC

  • RUBY_GC_HEAP_GROWTH_FACTOR - growth factor for slots

  • RUBY_GC_HEAP_GROWTH_MAX_SLOTS - maximum slots to add at one time

  • RUBY_GC_HEAP_FREE_SLOTS_MIN_RATIO - allocate additional slots when below this ratio

  • RUBY_GC_HEAP_FREE_SLOTS_MAX_RATIO - free pages of slots when above this ratio

  • RUBY_GC_HEAP_FREE_SLOTS_GOAL_RATIO - allocate slots to get to this ratio free (if 0.0, use the growth factor)

  • RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR - do a major GC when the number of old objects is above this factor times the old objects after the last major GC.








First: EnvMem assumes you're using the latest Ruby or something close to it. If you want to improve speed and memory use in Ruby and you're running an old version, fix that first.

There are a lot of things you can do with the Ruby environment variables, and many different applications with different needs. Right now, EnvMem tries to do a bit to help you. But there's always room for more.

(You can view these as limitations in EnvMem. You can also view them as places you can begin optimization. Both are correct.)

For instance:

EnvMem doesn't try to preserve environment variable settings from before you ran it. If you changed any of the "growth factors," for instance, EnvMem won't currently change them. You may also want to reduce the growth factors for a fully mature application, or set some of the LIMIT_MAX environment variables so that your app can't bloat as quickly. EnvMem won't do that for you either since it's so application-specific what "reasonable" behavior is.

EnvMem also tries not to assert anything about the balance of old- and new-generation objects. In a tightly-optimized application you'd expect old objects to dominate, while an application that generates a lot of transient garbage may need different settings. It's possible to balance MALLOC_LIMIT settings with OLDMALLOC_LIMIT settings to affect this, but EnvMem doesn't try to.

Similarly, you may want a much smaller FREE_SLOTS ratio with a more mature, more tightly-tuned application. EnvMem doesn't look at this, either.


After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open source under the terms of the MIT License.


A simple memory tool to save memory configuration from a mature process to fast-start a newer instance of that process.



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