Track memory allocated during a transaction

This gives a very rough estimate of how much memory is allocated during
a transaction. This only works reliably when using a single-threaded
application server and a Ruby implementation with a GIL as otherwise
memory allocated by other threads might skew the statistics. Sadly
there's no way around this as Ruby doesn't provide a reliable way of
gathering accurate object sizes upon allocation on a per-thread basis.

lib/gitlab/metrics/transaction.rb

@@ -23,19 +23,28 @@ module Gitlab
         @values = Hash.new(0)
         @tags   = {}
         @action = action
+
+        @memory_before = 0
+        @memory_after  = 0
       end
 
       def duration
         @finished_at ? (@finished_at - @started_at) * 1000.0 : 0.0
       end
 
+      def allocated_memory
+        @memory_after - @memory_before
+      end
+
       def run
         Thread.current[THREAD_KEY] = self
 
+        @memory_before = System.memory_usage
         @started_at    = Time.now
 
         yield
       ensure
+        @memory_after = System.memory_usage
         @finished_at  = Time.now
 
         Thread.current[THREAD_KEY] = nil
@@ -65,7 +74,7 @@ module Gitlab
       end
 
       def track_self
-        values = { duration: duration }
+        values = { duration: duration, allocated_memory: allocated_memory }
 
         @values.each do |name, value|
           values[name] = value

spec/lib/gitlab/metrics/transaction_spec.rb

@@ -11,6 +11,14 @@ describe Gitlab::Metrics::Transaction do
     end
   end
 
+  describe '#allocated_memory' do
+    it 'returns the allocated memory in bytes' do
+      transaction.run { 'a' * 32 }
+
+      expect(transaction.allocated_memory).to be_a_kind_of(Numeric)
+    end
+  end
+
   describe '#run' do
     it 'yields the supplied block' do
       expect { |b| transaction.run(&b) }.to yield_control
@@ -43,8 +51,10 @@ describe Gitlab::Metrics::Transaction do
       transaction.increment(:time, 1)
       transaction.increment(:time, 2)
 
+      values = { duration: 0.0, time: 3, allocated_memory: a_kind_of(Numeric) }
+
       expect(transaction).to receive(:add_metric).
-        with('transactions', { duration: 0.0, time: 3 }, {})
+        with('transactions', values, {})
 
       transaction.track_self
     end
@@ -54,8 +64,14 @@ describe Gitlab::Metrics::Transaction do
     it 'sets a value' do
       transaction.set(:number, 10)
 
+      values = {
+        duration:         0.0,
+        number:           10,
+        allocated_memory: a_kind_of(Numeric)
+      }
+
       expect(transaction).to receive(:add_metric).
-        with('transactions', { duration: 0.0, number: 10 }, {})
+        with('transactions', values, {})
 
       transaction.track_self
     end
@@ -80,8 +96,13 @@ describe Gitlab::Metrics::Transaction do
 
   describe '#track_self' do
     it 'adds a metric for the transaction itself' do
+      values = {
+        duration:         transaction.duration,
+        allocated_memory: a_kind_of(Numeric)
+      }
+
       expect(transaction).to receive(:add_metric).
-        with('transactions', { duration: transaction.duration }, {})
+        with('transactions', values, {})
 
       transaction.track_self
     end
@@ -104,7 +125,7 @@ describe Gitlab::Metrics::Transaction do
       hash = {
         series:    'rails_transactions',
         tags:      { action: 'Foo#bar' },
-        values:    { duration: 0.0 },
+        values:    { duration: 0.0, allocated_memory: a_kind_of(Numeric) },
         timestamp: an_instance_of(Fixnum)
       }