From eea5f896780e07f7ca76685061d01e7be5a7abaa Mon Sep 17 00:00:00 2001
From: Lukas Larsson <lukas@erlang-solutions.com>
Date: Thu, 11 Sep 2014 18:26:26 +0200
Subject: erts, kernel: Add os:perf_counter function

The perf_counter is a very very cheap and high resolution timer
that can be used to timestamp system events. It does not have
monoticity guarantees, but should on most OS's expose a monotonous
time.

A special instruction has been created for this counter to further
speed up fetching it.

OTP-12908
---
 lib/kernel/doc/src/os.xml | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

(limited to 'lib/kernel/doc')

diff --git a/lib/kernel/doc/src/os.xml b/lib/kernel/doc/src/os.xml
index 682d4a2eac..165160a909 100644
--- a/lib/kernel/doc/src/os.xml
+++ b/lib/kernel/doc/src/os.xml
@@ -37,6 +37,7 @@
       use these functions can be of help in enabling a program to run on
       most platforms.</p>
   </description>
+
   <funcs>
     <func>
       <name name="cmd" arity="1"/>
@@ -209,6 +210,30 @@ format_utc_timestamp() ->
 	and <seealso marker="#system_time/1"><c>os:system_time/1</c></seealso>.</p>
       </desc>
     </func>
+    <func>
+      <name name="perf_counter" arity="0"/>
+      <fsummary>Returns a performance counter</fsummary>
+      <desc>
+        <p>Returns the current performance counter value in <c>perf_counter</c>
+	<seealso marker="erts:erlang#type_time_unit">time unit</seealso>.
+        This is a highly optimized call that might not be traceable.
+        </p>
+      </desc>
+    </func>
+    <func>
+      <name name="perf_counter" arity="1"/>
+      <fsummary>Returns a performance counter</fsummary>
+      <desc><p>Returns a performance counter that can be used as a very fast and
+      high resolution timestamp. This counter is read directly from the hardware or operating
+      system with the same guarantees. This means that two consecutive calls
+      to the function are not guaranteed to be monotonic, though it most likely will be.
+      The performance counter till be converted to the resolution passed as an argument.</p>
+      <pre>1> <input>T1 = os:perf_counter(1000),receive after 10000 -> ok end,T2 = os:perf_counter(1000).</input>
+176525861
+2> <input>T2 - T1.</input>
+10004</pre>
+      </desc>
+    </func>
     <func>
       <name name="type" arity="0"/>
       <fsummary>Return the OS family and, in some cases, OS name of the current operating system</fsummary>
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