Why Rails App Memory Bloat Happens: Causes and Solutions (2025 Edition)

After running a Rails application in production for a while, you may encounter a phenomenon where memory usage grows unexpectedly large. In July 2025, I investigated the causes of this behavior and explored countermeasures. Here is a summary of my findings. The reason a Rails app "appears to keep consuming memory" is that glibc, which Ruby relies on, retains freed memory internally for future reuse rather than returning it to the OS. This is distinct from a typical memory leak. Starting with Ruby 3.3.0, you can optimize the heap by calling Process.warmup after the Rails application has finished booting. However, since this mechanism is intended to be executed at the completion of the application boot sequence, it is not easily applicable to reducing memory in long-running Rails applications. As a countermeasure that requires zero changes to product code, setting the environment variable MALLOC_ARENA_MAX=2 remains effective. This prevents glibc from creating numerous arenas (memory pool