idle-gc

IdleGC makes it easy to automatically run garbage collection when your Crystal process is otherwise idle. Idle-time garbage collection is beneficial because:

1. It reduces the memory size of the program, reducing operating costs, and
2. It may prevent the garbage collector from running during a latency-sensitive period.

IdleGC is easy-to-use. Simply call IdleGC.start and a new background Fiber will continuously poll for idle periods, garbage collecting as needed.

If you have request-oriented processing, such as in a web server, you may additionally call IdleGC.background_collect near the end of your request cycle. This won't block your response from completing, but will only collect after the system is idle. It's perfectly safe to call IdleGC.background_collect multiple times, and it will only schedule a single collection if it hasn't had a chance to run yet.

IdleGC is currently used in production on Total Real Returns.

Benchmarks

It's hard to get reliable memory usage benchmarks. Test it in your own application.

Configuration	Heap Size (MiB)	Runtime (s)	Max Latency (ms)	Memory Savings (%)	Recommended?
(off)	588.79	9.90	153	(baseline)	_
Timer	532.82	9.90	154	-9.5%	_
Synchronous	369.60	9.90	151	-37.2%	_
Background	369.80	9.90	151	-37.2%	_
Timer + Synchronous	362.15	9.90	151	-38.5%	_
Timer + Background	361.73	9.90	151	-38.6%	RECOMMENDED
Timer + request_limit=2	579.28	9.90	167	-1.6%	_

(Timer means a periodic idle GC. Synchronous means a blocking IdleGC.collect call within the request processing. Background means calling IdleGC.background_collect within the request processing. Benchmarks are run via make bench within d_dev. Results shown are best-of-5.)

CRYSTAL_LOAD_DEBUG_INFO=0

You may want to set the environment variable CRYSTAL_LOAD_DEBUG_INFO=0 before running your Crystal-compiled binary. This prevents the Crystal runtime from loading debugging info when an Exception occurs and the runtime attempts to print a backtrace. The loading of debug info causes a one-time massive spike in memory usuage, which is not freed by garbage collection. The attemtped loading of debug info, and the memory spike, happens even when the binary was compiled with crystal build --no-debug, which strips out debug info.

For web servers built using kemal or similar, note that Exceptions happen even if the client simply disconnects before a response is sent. This does not indicate an issue with your server software, but will still cause a massive memory spike unless you have CRYSTAL_LOAD_DEBUG_INFO=0 set.

Of course, if you do set CRYSTAL_LOAD_DEBUG_INFO=0, you will not see method names in your backtrace.

This CRYSTAL_LOAD_DEBUG_INFO=0 setting is currently undocumented. You can find it in the Crystal code here: https://github.com/crystal-lang/crystal/blob/1.5.0/src/exception/call_stack/stackwalk.cr#L11

(This is only related to IdleGC because if you're reading this README, you likely want to avoid large sudden spikes in memory usage.)

Installation

1. Add the dependency to your shard.yml:

   dependencies:
     idle-gc:
       github: compumike/idle-gc

2. Run shards install

Usage

Timer-based polling:

require "idle-gc"

IdleGC.start

Background collection after a request:

def your_request_handler(context : HTTP::Server::Context)
   ... your code here...

   IdleGC.background_collect
end

Development

Run ./d_dev to bring up a docker container for development, where you can easily run:

make spec     # to run unit tests
make bench    # to run benchmarks

Author

• compumike - creator and maintainer