1 files changed, 40 insertions, 0 deletions
diff --git a/spec/lib/gitlab/metrics/memory_spec.rb b/spec/lib/gitlab/metrics/memory_spec.rb
new file mode 100644
index 00000000000..fd8ca3b37c6
--- /dev/null
+++ b/spec/lib/gitlab/metrics/memory_spec.rb
@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+
+RSpec.describe Gitlab::Metrics::Memory do
+  describe '.gc_heap_fragmentation' do
+    subject(:call) do
+      described_class.gc_heap_fragmentation(
+        heap_live_slots: gc_stat_heap_live_slots,
+        heap_eden_pages: gc_stat_heap_eden_pages
+      )
+    end
+
+    context 'when the Ruby heap is perfectly utilized' do
+      # All objects are located in a single heap page.
+      let(:gc_stat_heap_live_slots) { described_class::HEAP_SLOTS_PER_PAGE }
+      let(:gc_stat_heap_eden_pages) { 1 }
+
+      it { is_expected.to eq(0) }
+    end
+
+    context 'when the Ruby heap is greatly fragmented' do
+      # There is one object per heap page.
+      let(:gc_stat_heap_live_slots) { described_class::HEAP_SLOTS_PER_PAGE }
+      let(:gc_stat_heap_eden_pages) { described_class::HEAP_SLOTS_PER_PAGE }
+
+      # The heap can never be "perfectly fragmented" because that would require
+      # zero objects per page.
+      it { is_expected.to be > 0.99 }
+    end
+
+    context 'when the Ruby heap is semi-fragmented' do
+      # All objects are spread over two pages i.e. each page is 50% utilized.
+      let(:gc_stat_heap_live_slots) { described_class::HEAP_SLOTS_PER_PAGE }
+      let(:gc_stat_heap_eden_pages) { 2 }
+
+      it { is_expected.to eq(0.5) }
+    end
+  end
+end