DEV: Add site setting to allow collection of GC stats during requests (#22898)

What does this change do?

This change adds a hidden `track_gc_stat_per_request` site setting which
when enabled will track the time spent in GC, major GC count and minor
GC count during a request.

Why is this change needed?

We have plans to tune our GC in production but without any
instrumentation, we will not be able to know if our tuning is effective
or not. This commit takes the first step at instrumenting some basic GC
stats in core during a request which can then be consumed by the discourse-prometheus plugin.

config/site_settings.yml

@@ -2148,6 +2148,9 @@ developer:
   max_sidebar_section_links:
     default: 50
     hidden: true
+  track_gc_stat_per_request:
+    default: false
+    hidden: true
 
 navigation:
   navigation_menu:

lib/method_profiler.rb

@@ -89,8 +89,13 @@ class MethodProfiler
   end
 
   def self.start(transfer = nil)
-    Thread.current[:_method_profiler] = transfer ||
+    Thread.current[:_method_profiler] = if transfer
-      { __start: Process.clock_gettime(Process::CLOCK_MONOTONIC) }
+      transfer
+    else
+      data = { __start: Process.clock_gettime(Process::CLOCK_MONOTONIC) }
+      data[:gc_stat] = GC.stat if SiteSetting.track_gc_stat_per_request
+      data
+    end
   end
 
   def self.clear
@@ -99,11 +104,22 @@ class MethodProfiler
 
   def self.stop
     finish = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+
     if data = Thread.current[:_method_profiler]
       Thread.current[:_method_profiler] = nil
       start = data.delete(:__start)
       data[:total_duration] = finish - start
+
+      if start_gc_stat = data.delete(:gc_stat)
+        end_gc_stat = GC.stat
+
+        data[:gc] ||= {}
+        data[:gc][:time] = (end_gc_stat[:time] - start_gc_stat[:time]) / 1000.0
+        data[:gc][:major_count] = end_gc_stat[:major_gc_count] - start_gc_stat[:major_gc_count]
+        data[:gc][:minor_count] = end_gc_stat[:minor_gc_count] - start_gc_stat[:minor_gc_count]
+      end
     end
+
     data
   end
 

spec/lib/method_profiler_spec.rb

@@ -37,4 +37,24 @@ RSpec.describe MethodProfiler do
 
     expect(result[:at_beach][:calls]).to eq(2)
   end
+
+  it "profiles GC stat information when `track_gc_stat_per_request` site setting has been enabled" do
+    MethodProfiler.start
+    GC.start(full_mark: false) # Minor GC
+    result = MethodProfiler.stop
+
+    expect(result[:gc]).not_to be_present
+
+    SiteSetting.track_gc_stat_per_request = true
+
+    MethodProfiler.start
+    GC.start(full_mark: true) # Major GC
+    GC.start(full_mark: false) # Minor GC
+    result = MethodProfiler.stop
+
+    expect(result[:gc]).to be_present
+    expect(result[:gc][:time]).to be >= 0.0
+    expect(result[:gc][:major_count]).to eq(1)
+    expect(result[:gc][:minor_count]).to eq(1)
+  end
 end