Use clock_gettime for all performance timestampsperformance-clock-adjustments

Process.clock_gettime allows getting the real time in nanoseconds as
well as allowing one to get a monotonic timestamp. This offers greater
accuracy without the overhead of having to allocate a Time instance. In
general using Time.now/Time.new is about 2x slower than using
Process.clock_gettime(). For example:

    require 'benchmark/ips'

    Benchmark.ips do |bench|
      bench.report 'Time.now' do
        Time.now.to_f
      end

      bench.report 'clock_gettime' do
        Process.clock_gettime(Process::CLOCK_MONOTONIC, :millisecond)
      end

      bench.compare!
    end

Running this benchmark gives:

    Calculating -------------------------------------
                Time.now   108.052k i/100ms
           clock_gettime   125.984k i/100ms
    -------------------------------------------------
                Time.now      2.343M (± 7.1%) i/s -     11.670M
           clock_gettime      4.979M (± 0.8%) i/s -     24.945M

    Comparison:
           clock_gettime:  4979393.8 i/s
                Time.now:  2342986.8 i/s - 2.13x slower

Another benefit of using Process.clock_gettime() is that we can simplify
the code a bit since it can give timestamps in nanoseconds out of the
box.

1 files changed, 14 insertions, 2 deletions

diff --git a/spec/lib/gitlab/metrics/system_spec.rb b/spec/lib/gitlab/metrics/system_spec.rb
index d6ae54e25e8..cf0e282c2fb 100644
--- a/spec/lib/gitlab/metrics/system_spec.rb
+++ b/spec/lib/gitlab/metrics/system_spec.rb
@@ -28,8 +28,20 @@ describe Gitlab::Metrics::System do
   end
 
   describe '.cpu_time' do
-    it 'returns a Fixnum' do
-      expect(described_class.cpu_time).to be_an_instance_of(Fixnum)
+    it 'returns a Float' do
+      expect(described_class.cpu_time).to be_an_instance_of(Float)
+    end
+  end
+
+  describe '.real_time' do
+    it 'returns a Float' do
+      expect(described_class.real_time).to be_an_instance_of(Float)
+    end
+  end
+
+  describe '.monotonic_time' do
+    it 'returns a Float' do
+      expect(described_class.monotonic_time).to be_an_instance_of(Float)
     end
   end
 end