Randomize metrics sample intervals

Sampling data at a fixed interval means we can potentially miss data from events occurring between sampling intervals. For example, say we sample data every 15 seconds but Unicorn workers get killed after 10 seconds. In this particular case it's possible to miss interesting data as the sampler will never get to actually submitting data. To work around this (at least for the most part) the sampling interval is randomized as following: 1. Take the user specified sampling interval (15 seconds by default) 2. Divide it by 2 (referred to as "half" below) 3. Generate a range (using a step of 0.1) from -"half" to "half" 4. Every time the sampler goes to sleep we'll grab the user provided interval and add a randomly chosen "adjustment" to it while making sure we don't pick the same value twice in a row. For a specified timeout of 15 this means the actual intervals can be anywhere between 7.5 and 22.5, but never can the same interval be used twice in a row. The rationale behind this change is that on dev.gitlab.org I'm sometimes seeing certain Gitlab::Git/Rugged objects being retained, but only for a few minutes every 24 hours. Knowing the code of Gitlab and how much memory it uses/leaks I suspect we're missing data due to workers getting terminated before the sampler can write its data to InfluxDB.

Randomize metrics sample intervals
057eb824 · Yorick Peterse · 23671600 · 057eb824 · 057eb824
Commit 057eb824 authored Jan 13, 2016 by Yorick Peterse
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Showing with 46 additions and 4 deletions

sampler.rb lib/gitlab/metrics/sampler.rb +25 -3

sampler_spec.rb spec/lib/gitlab/metrics/sampler_spec.rb +21 -1

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--- a/lib/gitlab/metrics/sampler.rb
+++ b/lib/gitlab/metrics/sampler.rb
@@ -8,8 +8,13 @@ module Gitlab
    class Sampler
      # interval - The sampling interval in seconds.
      def initialize(interval = Metrics.settings[:sample_interval])
-        @interval = interval
-        @metrics  = []
+        interval_half = interval.to_f / 2
+
+        @interval       = interval
+        @interval_steps = (-interval_half..interval_half).step(0.1).to_a
+        @last_step      = nil
+
+        @metrics = []

        @last_minor_gc = Delta.new(GC.stat[:minor_gc_count])
        @last_major_gc = Delta.new(GC.stat[:major_gc_count])
@@ -26,7 +31,7 @@ module Gitlab
          Thread.current.abort_on_exception = true

          loop do
-            sleep(@interval)
+            sleep(sleep_interval)

            sample
          end
@@ -102,6 +107,23 @@ module Gitlab
      def sidekiq?
        Sidekiq.server?
      end
+
+      # Returns the sleep interval with a random adjustment.
+      #
+      # The random adjustment is put in place to ensure we:
+      #
+      # 1. Don't generate samples at the exact same interval every time (thus
+      #    potentially missing anything that happens in between samples).
+      # 2. Don't sample data at the same interval two times in a row.
+      def sleep_interval
+        while step = @interval_steps.sample
+          if step != @last_step
+            @last_step = step
+
+            return @interval + @last_step
+          end
+        end
+      end
    end
  end
 end
--- a/spec/lib/gitlab/metrics/sampler_spec.rb
+++ b/spec/lib/gitlab/metrics/sampler_spec.rb
@@ -9,7 +9,7 @@ describe Gitlab::Metrics::Sampler do

  describe '#start' do
    it 'gathers a sample at a given interval' do
-      expect(sampler).to receive(:sleep).with(5)
+      expect(sampler).to receive(:sleep).with(a_kind_of(Numeric))
      expect(sampler).to receive(:sample)
      expect(sampler).to receive(:loop).and_yield

@@ -116,4 +116,24 @@ describe Gitlab::Metrics::Sampler do
      sampler.add_metric('cats', value: 10)
    end
  end
+
+  describe '#sleep_interval' do
+    it 'returns a Numeric' do
+      expect(sampler.sleep_interval).to be_a_kind_of(Numeric)
+    end
+
+    # Testing random behaviour is very hard, so treat this test as a basic smoke
+    # test instead of a very accurate behaviour/unit test.
+    it 'does not return the same interval twice in a row' do
+      last = nil
+
+      100.times do
+        interval = sampler.sleep_interval
+
+        expect(interval).to_not eq(last)
+
+        last = interval
+      end
+    end
+  end
 end