Understanding Ruby Memory Behavior

How Ruby Manages Memory

Ruby uses a conservative mark-and-sweep garbage collector (GC), enhanced in newer versions with generational GC and compaction. However, Ruby's GC is not fully precise—it cannot always identify unreachable memory in certain edge cases, especially in C extensions or dynamic metaprogramming-heavy codebases.

Signs of a Memory Leak

  • Resident Set Size (RSS) grows indefinitely under stable load
  • GC runs more frequently but frees little memory
  • Latency increases without changes to workload
  • Heroku or Kubernetes OOMKills of worker or app processes

Root Causes of Ruby Memory Leaks

1. Retained Objects via Global or Class Variables

Large objects or request-specific data stored in class-level hashes or global variables can unintentionally persist across requests, especially in multi-threaded or singleton-based designs.

2. C Extensions with Manual Allocation

Many gems include native extensions (e.g., Nokogiri, PG) that allocate memory in C. If these do not register finalizers properly or fail to release memory explicitly, Ruby's GC cannot manage them.

3. Thread Leaks in Concurrent Ruby

Improperly terminated threads or workers (e.g., in Sidekiq or Puma) can hold on to memory and objects, especially if threads are created dynamically in uncontrolled ways.

4. Procs, Closures, and Metaprogramming

Lambdas or blocks retaining references to outer scope variables can pin large structures in memory if cached or passed around long-term (e.g., memoization gone wrong).

Diagnosing Ruby Memory Leaks

Step-by-Step Diagnostic Flow

  1. Establish a baseline: Use ps, top, or htop to measure RSS across worker processes.
  2. Install memory profiler: Use gems like memory_profiler, derailed_benchmarks, or stackprof.
  3. Trigger a request repeatedly: Create a synthetic workload or run integration tests with wrk or ab.
  4. Track object allocations: Compare GC stats before and after workload. Use GC.stat or ObjectSpace.
  5. Dump and analyze heap: Use heap_dump or objspace for visual inspection (e.g., with qcachegrind).

Example: Using memory_profiler

require 'memory_profiler'
report = MemoryProfiler.report do
  1000.times { MyApp.handle_request(synthetic_input) }
end
report.pretty_print(to_file: 'memory_report.txt')

Common Pitfalls

  • Using class variables to cache request data (e.g., @@user_data)
  • Relying on weak reference gems without understanding finalization
  • Assuming GC will clean up all unused memory automatically
  • Ignoring memory growth in background jobs or daemon threads

Fixes and Long-Term Solutions

1. Limit Object Lifetime

Use local variables over global or class-scoped ones. Ensure large data structures are dereferenced (e.g., @large_array = nil) after use in long-lived classes or workers.

2. Monitor Memory with External Tools

Integrate Prometheus, New Relic, or Scout to track process-level memory usage. Set thresholds to trigger container restarts or alerts.

3. Restart Worker Processes Periodically

For long-running processes (e.g., Sidekiq, Puma), use phased or rolling restarts to limit memory growth. Tools like puma_worker_killer automate this.

4. Upgrade Ruby Version and Tune GC

Ruby 3.x includes compaction and better GC performance. Tune RUBY_GC_HEAP_GROWTH_FACTOR and RUBY_GC_MALLOC_LIMIT for high-traffic systems.

Best Practices

  • Use heap dumps during staging load tests to catch leaks early
  • Isolate third-party gems with memory-intensive behaviors
  • Avoid retaining large objects in lambdas or class constants
  • Log GC time and object count in production logs periodically

Conclusion

Memory leaks in Ruby may not crash an application immediately but can degrade performance and reliability over time—especially in always-on environments. By combining memory profiling, architectural discipline, and GC tuning, teams can prevent leak patterns from affecting end-user experience. Clear separation of object lifetimes, routine memory audits, and upgraded runtime tooling ensure Ruby applications remain performant and production-hardened even at scale.

FAQs

1. Why does my Ruby app consume more memory over time?

Common causes include retained objects, memory leaks in C extensions, or threads holding references. GC cannot reclaim such memory.

2. How do I generate a heap dump in Ruby?

Use the objspace or heap_dump gems. These provide snapshots of memory allocations for offline analysis.

3. Are background jobs more prone to leaks?

Yes. Long-lived Sidekiq workers or threads may retain memory longer unless explicitly cleared or restarted periodically.

4. Does Ruby automatically free all unused memory?

No. Ruby GC may miss certain objects, especially with C extensions or complex closures. Manual dereferencing is often necessary.

5. Is upgrading Ruby enough to fix memory leaks?

Upgrading helps due to GC improvements, but structural code fixes are required for leaks caused by poor memory management or misuse of global scope.