Explain this formula.

1 files changed, 13 insertions, 0 deletions

diff --git a/src/rabbit_misc.erl b/src/rabbit_misc.erl
index 2e87a8f6e1..58e93a3f9e 100644
--- a/src/rabbit_misc.erl
+++ b/src/rabbit_misc.erl
@@ -1059,6 +1059,19 @@ store_proc_name(TypeProcName)   -> put(process_name, TypeProcName).
 
 moving_average(_Time, _HalfLife, Next, undefined) ->
     Next;
+%% We want the Weight to decrease as Time goes up (since Weight is the
+%% weight for the current sample, not the new one), so that the moving
+%% average decays at the same speed regardless of how long the time is
+%% between samplings. So we want Weight = math:exp(Something), where
+%% Something turns out to be negative.
+%%
+%% We want to determine Something here in terms of the Time taken
+%% since the last measurement, and a HalfLife. So we want Weight =
+%% math:exp(Time * Constant / HalfLife). What should Constant be? We
+%% want Weight to be 0.5 when Time = HalfLife.
+%%
+%% Plug those numbers in and you get 0.5 = math:exp(Constant). Take
+%% the log of each side and you get math:log(0.5) = Constant.
 moving_average(Time,  HalfLife,  Next, Current) ->
     Weight = math:exp(Time * math:log(0.5) / HalfLife),
     Next * (1 - Weight) + Current * Weight.