Speed up supervisor:count_children/1; simple_one_for_one

Speed up supervisor:count_children/1 for simple_one_for_one
supervisors. This is achieved by avoiding looping through all the
child process and verifying that each one is alive.

For a supervisor with 100,000 'temporary' children the count-time will
drop from approx 25ms to about 0.005ms.

For a supervisor with 100,000 'permanent' or 'transient' children the
count-time will drop from approx 30ms to about 0.005ms.

This avoids having the supervisor block for an extended period while
the count takes place. Under normal circumstances the accuracy of the
result should also improve since the duration is too short for many
processes to die during the count.

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/stdlib/doc/src/supervisor.xml b/lib/stdlib/doc/src/supervisor.xml
index 24ff251ce3..815bf4a489 100644
--- a/lib/stdlib/doc/src/supervisor.xml
+++ b/lib/stdlib/doc/src/supervisor.xml
@@ -543,7 +543,10 @@
           </item>
           <item>
             <p><c>active</c> - the count of all actively running child processes
-              managed by this supervisor.</p>
+              managed by this supervisor. In the case of <c>simple_one_for_one</c>
+              supervisors, no check is carried out to ensure that each child process
+              is still alive, though the result provided here is likely to be very
+              accurate unless the supervisor is heavily overloaded.</p>
           </item>
           <item>
             <p><c>supervisors</c> - the count of all children marked as