[STRATOS-939] CEP sends very large values for gradient and second derivative of load average

• Type: Bug
• Status: Resolved
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: 4.1.0 M3
• Fix Version/s: 4.1.0 RC3
• Component/s: CEP
• Assignee: Nirmal Fernando
• Reporter: Nirmal Fernando
• Created: 05/Nov/14 20:22
• Updated: 31/May/15 07:17
• Resolved: 05/Nov/14 20:35

Description

How we calculate gradient of two events?

Say the events are; e1(t1,v1) and e2(t2,v2)

tx - time in milliseconds when xth event occurred
vx - value (memory, cpu etc.) that xth event carries

time gap = t(2-1) = t2 -t1 milliseconds
time gap in seconds = t(2-1) = (t2 - t1)/1000 seconds

Hence,
Gradient = (v2 - v1) / t(2-1) = ( (v2 - v1) * 1000 ) / (t2 - t1)

I've enabled debug logs for CEP extension;
log4j.logger.org.apache.stratos.cep.extension=DEBUG

Please find the following 3 logs extracted from the debug logs;

===================================================================
TID: [0] [STRATOS] [2014-11-05 19:47:27,073] DEBUG

{org.apache.stratos.cep.extension.SecondDerivativeFinderWindowProcessor} - Gradient: -0.1996007984031936 Last val: 9.0 First val: 12.0 Time Gap: 15030 t1: 1415213202095 t2: 1415213217125 hash: 155426542

TID: [0] [STRATOS] [2014-11-05 19:47:27,073] DEBUG {org.apache.stratos.cep.extension.SecondDerivativeFinderWindowProcessor}

- Gradient: -999.9999999999998 Last val: 7.000000000000001 First val: 12.0 Time Gap: 5 t1: 1415213232152 t2: 1415213232157 hash: 155426542

TID: [0] [STRATOS] [2014-11-05 19:47:27,074] DEBUG

{org.apache.stratos.cep.extension.SecondDerivativeFinderWindowProcessor}

- Gradient: -44.34884666437174 Last val: -999.9999999999998 First val: -0.1996007984031936 Time Gap: 22544 t1: 1415213209610 t2: 1415213232154 hash: 155426542
===================================================================

So, as you can see the reason behind a large value is when the time gap between two subjected events is less than 1 second. This could happen since events are coming from different asynchronous agents and also when there are less number of events.

FIX
====

So, the fix I propose is a very simple one and it will not compromise anything AFAIS.

Fix is to calculate time gap as follows;

time gap = t(2-1) } t2 -t1 > 1000 -----> t2 - t1
t2 - t1 <= 1000 ----> 1000

I have tested this and works fine.