The Silent Siege: An Artisan's Guide to Hunting Memory Leaks in Long-Running Rails Processes

[Alex Aslam]

You’ve built a masterpiece. It’s a complex, elegant Rails service—a data hydra, a real-time event processor, a background canvas painter. It runs not in the brief, transactional bursts of a web request, but as a long-running daemon, a silent guardian in your system’s architecture.

For a while, everything is sublime. It hums along, efficient and potent. But then, you start to see it. Not a crash, not an error. A slow, insidious creep. The resident set size (RSS) chart, once a flat, tranquil lake, begins to look like a set of stairs leading to the sky. Each restart is a temporary reprieve, a sigh of relief before the inevitable climb begins anew.

Your guardian is under a silent siege. It has a memory leak.

This isn’t a bug to be fixed with a quick if statement. This is a hunt. And for senior developers, hunting a leak is less a science and more a dark art—a meticulous form of performance artistry. Let this be your guide to becoming an artisan leak-hunter.

The Canvas: Understanding the Landscape of Memory in Ruby​

Before we pick up our tools, we must understand our medium. Ruby’s memory isn't a blank, linear array; it's a rich, complex tapestry woven with two primary threads:

1. The Heap: Imagine a vast grid of slots, each capable of holding a reference to an object (a String, an Array, a custom User instance). This is managed by Ruby's Garbage Collector (GC).
2. ️ The Garbage Collector (The Janitor): The GC is a meticulous, if sometimes overworked, janitor. It periodically sweeps through the heap, marking objects that are still "reachable" (referenced somewhere in your code) and freeing the slots holding "unreachable" ones.

A memory leak, in its purest form, occurs when you continuously create objects that become unreachable but are never marked for collection. You are filling the janitor’s closets with boxes he’s forgotten how to open.

In a long-running process, even a tiny leak—a few KB per job—compounds into a Gigabyte-sized catastrophe. Our quest is to find what’s holding those references hostage.

The Palette: Our Arsenal of Tools​

An artist is nothing without their tools. We will not be guessing. We will be measuring.

Tool	Purpose	The Artistic Analogy
GC.stat	A built-in Ruby method that returns a hash of vital statistics about the current state of the garbage collector.	The ground-level survey. Checking the lay of the land.
ObjectSpace	A module to interact with all living objects in the Ruby heap. Powerful, but handle with care.	The microscope. Allows for intense, precise examination.
memory_profiler gem	A brilliant gem that can take a snapshot of memory usage before and after a block of code, detailing object allocation and retention.	The time-lapse camera. Perfect for identifying trends.
derailed_benchmarks gem	Designed for apps, but its mem tasks can be adapted to profile specific code paths in isolation.	The stress-testing rig.
rbtrace gem	Attach to a live, running process and execute commands to see what’s happening inside. Essential for production debugging.	The psychic link to your running process.

The Masterpiece: A Step-by-Step Journey of Discovery​

1. The Groundwork: Establishing a Baseline​

First, we must quantify the problem. We can't know we've fixed it if we can't measure it.

Inside your process, likely in a loop, log the most critical metrics:

# In your process loop, perhaps every 100 iterations...
if (iteration_count % 100).zero?
  mem_stats = {
    rss: `ps -o rss= -p #{Process.pid}`.to_i / 1024, # Resident Set Size in MB
    heap_live_slots: GC.stat(:heap_live_slots),
    heap_free_slots: GC.stat(:heap_free_slots),
    total_allocated_objects: GC.stat(:total_allocated_objects)
  }
  Rails.logger.info("[MEM-STATS] #{mem_stats.to_json}")
end

This log will paint the picture of the siege. Is rss and heap_live_slots climbing in lockstep? You have a true object leak. Is rss climbing while heap_live_slots plateaus? You may have a leak in a native C extension (a rarer, more sinister beast).

2. The Isolation: Reproducing in Development​

A hunt in production is a dangerous game. We must lure the beast into a controlled environment.

Create a script, bin/profile_leak.rb, that simulates the core workload of your daemon. This is your studio, where you can experiment without consequence.

# bin/profile_leak.rb
require 'memory_profiler'
require_relative '../config/environment' # Load your Rails env

report = MemoryProfiler.report do
  # Simulate the core work of your long-running process.
  100.times do
    MyWorkerService.perform_core_task
  end
end

# Generate a detailed report showing retained objects
report.pretty_print(to_file: 'tmp/memory_profiler.txt')

Run this. The report will show you which classes are being retained in memory after the block finishes. Look for lines with high retained counts. This is your first major clue.

3. The Interrogation: rbtrace and the Live Process​

Sometimes the leak is environmental, only appearing under the specific pressures of production. This is where rbtrace becomes our most powerful tool.

• Add gem 'rbtrace' to your Gemfile.
• Deploy.
• 
  When the process's memory begins to bloat, attach to it:
  
  

  bundle exec rbtrace -p <PID> -e 'GC.stat'
  # Or, even more powerful, get a histogram of object types:
  bundle exec rbtrace -p <PID> -e 'ObjectSpace.each_object.group_by(&:class).map { |k,v| [k, v.count] }.to_h'

This command will output a list of all classes in memory and their counts. Compare this output from a freshly started process versus a bloated one. What class is disproportionately large? String? Array? Your own MyCustomJob class? You've now identified the "what."

4. The Culprits: Common Causes for the Seasoned Eye​

Knowing the "what" leads us to the "why." Here are the usual suspects we've all learned to distrust:

• 
  The Global Cache Trap: Constants and class variables are eternal. A cache like MyClass::CACHE ||= {} that never expires or is keyed by something infinite (like time) will grow forever.
  
  • The Artisan's Fix: Use a size-bound, LRU (Least Recently Used) cache like the lru_redux gem.
• 
  The Unbound Method Return: A method that returns an ever-growing array (e.g., User.all.map(&:name)). Each call appends more data to the live heap.
  
  • The Artisan's Fix: Use pagination or iterate with find_each to process in batches, keeping the object footprint small and constant.
• 
  The Anonymous Closure's Captive: Blocks (closures) capture their entire surrounding scope. A method that defines a Proc and stores it in a long-lived object can accidentally hold onto a huge scope it doesn't need.
  
  • The Artisan's Fix: Be mindful of scope. Extract the needed variables explicitly instead of capturing the entire environment.
• 
  The Thread's Lingering Luggage: Threads can be hard to kill cleanly. If a thread dies but the objects it was working on are still referenced from a global queue, they can be kept alive.
  
  • The Artisan's Fix: Implement robust thread lifecycle management and ensure job queues are properly cleared.

The Final Brushstrokes: Validation and Vigilance​

You’ve identified a suspect and applied a fix. Now, return to your baseline. Re-run your bin/profile_leak.rb script. The retained count for the offending class should be near zero.

Deploy to a staging environment and watch the memory chart. The once-climbing staircase should now be a flat, horizontal line—a calm horizon. You haven't just fixed a bug; you've restored balance.

The Gallery: A Never-Ending Exhibition​

Remember, optimizing memory in Ruby is not about achieving zero allocation. It's about achieving zero retention. Objects should live, serve their purpose, and be gracefully collected, leaving the heap ready for the next wave of work.

The journey of the artisan leak-hunter is cyclical, not linear. It’s a practice of vigilance, of understanding the deep interplay between your code and the Ruby runtime. It’s the art of writing code that is not just functional, but respectful of the system it inhabits.

Now go forth. Your silent guardian awaits its tune-up. May your charts be flat and your garbage collection cycles swift.

Continue reading...

 

Join 𝕋𝕄𝕋 on Telegram
Channel PREVIEW: