code-dot-org · sureshc · Jun 22, 2026 · Jun 23, 2026
diff --git a/lib/cdo/app_server_hooks.rb b/lib/cdo/app_server_hooks.rb
@@ -1,3 +1,5 @@
+require 'cdo/process_memory'
+
 module Cdo
   # NOTE: these hooks are only executed when running in puma clustered mode, which spawns worker processes.
   # These hooks will NOT be run in local development unless you set `dashboard_workers: 1` (or greater)
@@ -25,6 +27,42 @@ def self.before_fork
         restart_period = DCDO.get('web_service_process_restart_period', 12 * 3600) # default to 12 hours
         PumaWorkerKiller.enable_rolling_restart(restart_period)
       end
+
+      # Compact heap before forking child puma processes to reduce the number of heap pages occupied by long-lived
+      # objects, which reduces the surface area for Copy-on-Write erosion.
+      unless @compacted_heap_before_worker_fork
+        @compacted_heap_before_worker_fork = true
+
+        begin
+          before_gc = GC.stat
+          started_at = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+          Cdo::ProcessMemory.log_snapshot(
+            'Compacting Ruby heap before Puma worker fork',
+            fields: {
+              heap_allocated_pages: before_gc[:heap_allocated_pages],
+              heap_live_slots: before_gc[:heap_live_slots],
+              old_objects: before_gc[:old_objects]
+            }
+          )
+
+          GC.start(full_mark: true, immediate_sweep: true)
+          GC.compact
+
+          after_gc = GC.stat
+          duration = Process.clock_gettime(Process::CLOCK_MONOTONIC) - started_at
+          Cdo::ProcessMemory.log_snapshot(
+            'Compacted Ruby heap before Puma worker fork',
+            fields: {
+              duration_seconds: duration.round(3),
+              heap_allocated_pages: after_gc[:heap_allocated_pages],
+              heap_live_slots: after_gc[:heap_live_slots],
+              old_objects: after_gc[:old_objects]
+            }
+          )
+        rescue StandardError => exception
+          CDO.log.warn("Failed to compact Ruby heap before Puma worker fork: #{exception.class}: #{exception.message}")
+        end
+      end
     end
 
     def self.before_worker_boot(host:)

diff --git a/lib/cdo/process_memory.rb b/lib/cdo/process_memory.rb
@@ -0,0 +1,57 @@
+require 'cdo'
+
+module Cdo
+  # Lightweight process-memory snapshots for diagnostics.
+  #
+  # On Linux, this reads /proc/$pid/status and /proc/$pid/smaps_rollup and returns
+  # selected values in kilobytes. On macOS and other systems without those procfs
+  # files, it returns an empty hash. Missing or unreadable procfs files are
+  # treated as unavailable metrics, not as errors.
+  module ProcessMemory
+    STATUS_FIELDS = {
+      'VmRSS' => :proc_vm_rss_kb,
+      'VmHWM' => :proc_vm_hwm_kb,
+      'VmSize' => :proc_vm_size_kb,
+      'VmData' => :proc_vm_data_kb,
+      'VmSwap' => :proc_vm_swap_kb,
+      'Threads' => :proc_threads
+    }.freeze
+
+    SMAPS_ROLLUP_FIELDS = {
+      'Rss' => :smaps_rss_kb,
+      'Pss' => :smaps_pss_kb,
+      'Private_Clean' => :smaps_private_clean_kb,
+      'Private_Dirty' => :smaps_private_dirty_kb,
+      'Shared_Clean' => :smaps_shared_clean_kb,
+      'Shared_Dirty' => :smaps_shared_dirty_kb
+    }.freeze
+
+    def self.snapshot_kb(pid: Process.pid)
+      snapshot = {}
+      read_kb_fields("/proc/#{pid}/status", STATUS_FIELDS, snapshot)
+      read_kb_fields("/proc/#{pid}/smaps_rollup", SMAPS_ROLLUP_FIELDS, snapshot)
+      snapshot
+    end
+
+    def self.log_snapshot(message, fields: {}, pid: Process.pid)
+      metrics = fields.merge(snapshot_kb(pid: pid))
+      CDO.log.info("#{message}: #{metrics.map {|key, value| "#{key}=#{value}"}.join(', ')}")
+      metrics
+    end
+
+    def self.read_kb_fields(path, field_names, snapshot)
+      return snapshot unless File.readable?(path)
+
+      File.foreach(path) do |line|
+        key, value = line.split(':', 2)
+        metric_name = field_names[key]
+        snapshot[metric_name] = value.to_i if metric_name
+      end
+
+      snapshot
+    rescue Errno::ENOENT, Errno::EACCES
+      snapshot
+    end
+    private_class_method :read_kb_fields
+  end
+end