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authorBjörn-Egil Dahlberg <[email protected]>2012-03-07 17:11:16 +0100
committerBjörn-Egil Dahlberg <[email protected]>2012-03-07 17:11:16 +0100
commit1df6ba99c781fc8eeb30a02aab913d10600527ab (patch)
tree7bcb6d68f4d81792dc0351b23838200b54b62d31 /erts/doc/src
parenta7714f269498e51f02c598446ef8626c380d30b1 (diff)
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Update scheduler_wall_time documentation
* Add example on calculating scheduler utilization
Diffstat (limited to 'erts/doc/src')
-rw-r--r--erts/doc/src/erlang.xml52
1 files changed, 45 insertions, 7 deletions
diff --git a/erts/doc/src/erlang.xml b/erts/doc/src/erlang.xml
index fbe7b36163..ed79109bdd 100644
--- a/erts/doc/src/erlang.xml
+++ b/erts/doc/src/erlang.xml
@@ -4935,17 +4935,51 @@ true</pre>
</item>
<tag><marker id="statistics_scheduler_wall_time"><c>scheduler_wall_time</c></marker></tag>
<item>
- <p>Returns
- <c>[{Scheduler_Id, Scheduler_Worked_Time, Scheduler_Total_Time}]</c>, time lapses are since the
- the system flag <seealso marker="#system_flag_scheduler_wall_time">scheduler_wall_time</seealso>
- was set to true.
+ <p>Returns a list of tuples with
+ <c>{SchedulerId, ActiveTime, TotalTime}]</c>, where <c>SchedulerId</c> is an integer id of the scheduler, <c>ActiveTime</c> is
+ the duration the scheduler has been busy. <c>TotalTime</c> is the total time duration since
+ <seealso marker="#system_flag_scheduler_wall_time">scheduler_wall_time</seealso> activation. The time unit is not defined and may be subject to change. <c>scheduler_wall_time</c> should only be used to calculate relative values for scheduler-utilization. <c>ActiveTime</c> can never exceed <c>TotalTime</c>.
+ </p>
+
+ <p>The definition of a busy scheduler is when it is not idle or not scheduling (selecting) a process or port, meaning; executing process code, executing linked-in-driver or NIF code, garbage collecting or handling any other memory management.
+ </p>
+
+ <p>
Returns <c>undefined</c> if the system flag <seealso marker="#system_flag_scheduler_wall_time">
- scheduler_wall_time</seealso> is set to false.
+ scheduler_wall_time</seealso> is turned off.
</p>
+
<p>The list of scheduler information is unsorted and may come in different order
- between calls. The time unit is undefined and may be changed and should only be used
- to calculate relative utilization.
+ between calls.
</p>
+ <p>Using <c>scheduler_wall_time</c> to calculate scheduler utilization.</p>
+<pre>
+> <input>erlang:system_flag(scheduler_wall_time, true).</input>
+false
+> <input>Ts0 = lists:sort(erlang:statistics(scheduler_wall_time)), ok.</input>
+ok
+</pre>
+ <p>Some time later we will take another snapshot and calculate scheduler-utilization per scheduler.</p>
+<pre>
+> <input>Ts1 = lists:sort(erlang:statistics(scheduler_wall_time)), ok.</input>
+ok
+> <input>lists:map(fun({{I, W0, T0}, {I, W1, T1}}) -> {I, (W1 - W0)/(T1 - T0)} end, lists:zip(Ts0,Ts1)).</input>
+[{1,0.9743474730177548},
+ {2,0.9744843782751444},
+ {3,0.9995902361669045},
+ {4,0.9738012596572161},
+ {5,0.9717956667018103},
+ {6,0.9739235846420741},
+ {7,0.973237033077876},
+ {8,0.9741297293248656}]
+</pre>
+ <p>Using the same snapshot to calculate a total scheduler-utilization.</p>
+<pre>
+> <input>{W, T} = lists:foldl(fun({{_, W0, T0}, {_, W1, T1}}, {Wi,Ti}) ->
+ {Wi + (W1 - W0), Ti + (T1 - T0)} end, {0,0}, lists:zip(Ts0,Ts1)), W/T.</input>
+0.9769136803764825
+</pre>
+
</item>
<tag><c>wall_clock</c></tag>
@@ -4965,6 +4999,10 @@ true</pre>
{2046,11}
> <input>statistics(garbage_collection).</input>
{85,23961,0}</pre>
+ <note>
+ <p><c>scheduler_wall_time</c> is by default disabled. Use <c>erlang:system_flag(scheduler_wall_time, true)</c> to enable it. </p>
+ </note>
+
</desc>
</func>
<func>