Javieratapiab · March 29, 2020 04:52 · Mar 29, 2020
diff --git a/elapsed-time-ruby.rb b/elapsed-time-ruby.rb
@@ -0,0 +1,53 @@
+### THE WRONG WAY ###
+starting = Time.now
+# time consuming operation
+ending = Time.now
+elapsed = ending - starting
+elapsed # => 10.822178
+#####################
+
+### THE GOOD WAY ###
+starting = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+# time consuming operation
+ending = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+elapsed = ending - starting
+elapsed # => 9.183449000120163 seconds
+
+=begin
+Depending on the low level Operating System (OS) settings, 
+Ruby's Time.now uses `gettimeofday` or `clock_gettime` Linux functions from time.h.
+According to the documentation of gettimeofday:
+
+[It] gives the number of seconds and microseconds since the Epoch.
+
+It returns a struct with the number of seconds and the system time zone. 
+Ruby VM can then calculate and it returns a Time object with these informations.
+This is often indicated as wall time in Linux documentation.
+
+However:
+
+The time returned by `gettimeofday()` is affected by discontinuous jumps in the system time 
+(e.g., if the system administrator manually changes the system time).
+
+The lack of perfect accuracy is due to CPU physical conditions like temperature, 
+air pressure, and even magnetic fields.
+
+So when the OS tries to set a new system time, it doesn't guarantee that the new value will be in the future. 
+If a CPU has a clock that is "too fast", the OS can decide to reset the time of a few seconds backward.
+
+Another reason why wall clocks are flawed is because some CPUs can't manage leap seconds. 
+On Wikipedia there is a page dedicated to incidents in software history due to leap seconds.
+
+Another reason why wall clocks are flawed is because some CPUs can't manage leap seconds. 
+On Wikipedia there is a page dedicated to incidents in software history due to leap seconds.
+
+To recap: system clock is constantly floating and it doesn't move only forwards. 
+If your calculation of elapsed time is based on it, you're very likely to run into calculation errors or even outages.
+
+Posix systems have solved this problem by introducing a monotonic clock. 
+It's conceptually similar to a timer that starts with an event and it isn't affected by time floating problems. 
+Each time you request the time to the monotonic clock, it returns the time since that event. 
+On Mac OS, this event is the system boot. 
+Alongside with monotonic, there are several clock types: realtime, monotonic raw, virtual just to name a few. 
+Each of them solves a different problem.
+=end
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