Use clock_gettime for all performance timestamps

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, 18 insertions, 2 deletions

diff --git a/lib/gitlab/metrics/system.rb b/lib/gitlab/metrics/system.rb
index a7d183b2f94..82c18bb108b 100644
--- a/lib/gitlab/metrics/system.rb
+++ b/lib/gitlab/metrics/system.rb
@@ -34,13 +34,29 @@ module Gitlab
       # THREAD_CPUTIME is not supported on OS X
       if Process.const_defined?(:CLOCK_THREAD_CPUTIME_ID)
         def self.cpu_time
-          Process.clock_gettime(Process::CLOCK_THREAD_CPUTIME_ID, :millisecond)
+          Process.
+            clock_gettime(Process::CLOCK_THREAD_CPUTIME_ID, :millisecond).to_f
         end
       else
         def self.cpu_time
-          Process.clock_gettime(Process::CLOCK_PROCESS_CPUTIME_ID, :millisecond)
+          Process.
+            clock_gettime(Process::CLOCK_PROCESS_CPUTIME_ID, :millisecond).to_f
         end
       end
+
+      # Returns the current real time in a given precision.
+      #
+      # Returns the time as a Float.
+      def self.real_time(precision = :millisecond)
+        Process.clock_gettime(Process::CLOCK_REALTIME, precision).to_f
+      end
+
+      # Returns the current monotonic clock time in a given precision.
+      #
+      # Returns the time as a Float.
+      def self.monotonic_time(precision = :millisecond)
+        Process.clock_gettime(Process::CLOCK_MONOTONIC, precision).to_f
+      end
     end
   end
 end