RailsMemoryProfiler

Per-request memory allocation reports with a mountable dashboard UI. Fills the gap between the memory_profiler gem (terminal-only output, manual block wrapping) and having nowhere useful to browse results in a Rails app.

A Rack middleware captures object allocations for every request using GC.stat diffs. Results are stored in a thread-safe ring buffer and served through a mountable engine with a sortable, filterable dashboard.

Installation
Dashboard
Configuration
Compatibility
Contributing
License

Installation

Add to your Gemfile:

gem "rails_memory_profiler", group: :development

Run the install generator:

bundle exec rails generate rails_memory_profiler:install

This creates config/initializers/rails_memory_profiler.rb with all options documented and prints mount instructions.

Mount the dashboard in config/routes.rb:

mount RailsMemoryProfiler::Engine, at: "/rails/memory"

Then visit /rails/memory/reports to see per-request allocation data.

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Dashboard

The index view shows a sortable table of captured requests. Click any row to open the detail view for that request.

Columns: Path, Controller#Action, Allocated Objects (colour-coded), Retained Objects, Duration (ms), Recorded At.

Use the controller filter to narrow the table down to a specific controller without a page reload.

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Configuration

All options and their defaults:

Option	Default	Description
`enabled`	`true` in development	Enable/disable request profiling
`sample_rate`	`1`	Profile every Nth request (`1` = every request)
`store_size`	`100`	Max reports in the ring buffer; oldest are evicted when full
`dashboard_enabled`	`true` in development	Enable the dashboard endpoint
`min_allocated_objects`	`0`	Skip requests that allocate fewer objects than this
`ignore_paths`	`[]`	Additional paths to skip — strings (prefix) or regexes (the dashboard's own mount path is auto-ignored)
`ignore_controllers`	`[]`	Controller names to skip (e.g. `"rails/health"`)
`detailed_reports`	`false`	Capture full `MemoryProfiler.report` breakdowns; requires `gem "memory_profiler"` in your Gemfile
`detailed_sample_rate`	`10`	When `detailed_reports` is enabled, capture a full report every Nth profiled request
`notifiers`	`[]`	Array of objects responding to `#call(report)`; called after each report is stored
`log_file`	`nil`	Path to append JSON-serialized reports (one per line); shortcut for `Notifiers::FileLogger`
`raise_on_allocation_spike`	`nil`	Raise `AllocationSpikeError` when `allocated_objects` exceeds this threshold

Example:

RailsMemoryProfiler.configure do |config|
  config.sample_rate           = 5
  config.min_allocated_objects = 1_000
  config.ignore_paths          = ["/up"]   # dashboard mount path is auto-ignored
  config.ignore_controllers    = ["rails/health"]
  config.notifiers             = [RailsMemoryProfiler::Notifiers::Console.new]
  config.log_file              = Rails.root.join("log/memory_profiler.jsonl")
end

Test helpers

# Plain Ruby / shared utility
require "rails_memory_profiler/test_helper"

count = RailsMemoryProfiler::TestHelper.capture_allocations { MyClass.new }
RailsMemoryProfiler::TestHelper.assert_allocations_below(500) { MyClass.new }
# raises Minitest::Assertion when Minitest is loaded, RuntimeError otherwise

# Minitest — adds assert_allocates_fewer_than to all Minitest::Test subclasses
require "rails_memory_profiler/minitest_matchers"

assert_allocates_fewer_than(500) { MyClass.new }
assert_allocates_fewer_than(500, "MyClass allocates too much") { MyClass.new }

# RSpec
require "rails_memory_profiler/rspec_matchers"

expect { MyClass.new }.to allocate_fewer_than(500)

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Compatibility

	Ruby 3.3	Ruby 3.4	Ruby 4.0
Rails 7.1	✓	✓	✓
Rails 8.x	✓	✓	✓