overload(3erl)						     Erlang Module Definition						    overload(3erl)

NAME

       overload - An Overload Regulation Process

DESCRIPTION

       overload  is a process which indirectly regulates CPU usage in the system. The idea is that a main application calls the request/0 function
       before starting a major job, and proceeds with the job if the return value is positive; otherwise the job must not be started.

       overload is part of the sasl application, and all configuration parameters are defined there.

       A set of two intensities are maintained, the total intensity and the accept intensity . For that purpose there are two configuration param-
       eters, the MaxIntensity and the Weight value (both are measured in 1/second).

       Then  total  and accept intensities are calculated as follows. Assume that the time of the current call to request/0 is T(n) , and that the
       time of the previous call was T(n-1) .

	 * The current total intensity , denoted TI(n) , is calculated according to the formula,

	   TI(n) = exp(-Weight*(T(n) - T(n-1)) * TI(n-1) + Weight ,

	   where TI(n-1) is the previous total intensity.

	 * The current accept intensity , denoted AI(n) , is determined by the formula,

	   AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1) + Weight ,

	   where AI(n-1) is the previous accept intensity, provided that the value of exp(-Weight*(T(n) - T(n-1)) * AI(n-1) is less than MaxInten-
	   sity ; otherwise the value is

	   AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1) .

       The  value  of  configuration  parameter  Weight controls the speed with which the calculations of intensities will react to changes in the
       underlying input intensity. The inverted value of Weight ,

       T = 1/Weight

       can be thought of as the "time constant" of the intensity calculation formulas. For example, if Weight = 0.1 , then a change in the  under-
       lying input intensity will be reflected in the total and accept intensities within approximately 10 seconds.

       The overload process defines one alarm, which it sets using alarm_handler:set_alarm(Alarm) . Alarm is defined as:

	 {overload, []} :
	   This alarm is set when the current accept intensity exceeds MaxIntensity .

       A  new  overload  alarm	is not set until the current accept intensity has fallen below MaxIntensity . To prevent the overload process from
       generating a lot of set/reset alarms, the alarm is not reset until the current accept intensity has fallen below 75% of MaxIntensity ,  and
       it is not until then that the alarm can be set again.

EXPORTS

       request() -> accept | reject

	      Returns accept or reject depending on the current value of the accept intensity.

	      The  application	calling  this  function  should be processed with the job in question if the return value is accept ; otherwise it
	      should not continue with that job.

       get_overload_info() -> OverloadInfo

	      Types  OverloadInfo  =  [{total_intensity,  TotalIntensity},  {accept_intensity,	AcceptIntensity},  {max_intensity,  MaxIntensity},
		     {weight, Weight}, {total_requests, TotalRequests}, {accepted_requests, AcceptedRequests}].
		     TotalIntensity = float() > 0
		     AcceptIntensity = float() > 0
		     MaxIntensity = float() > 0
		     Weight = float() > 0
		     TotalRequests = integer()
		     AcceptedRequests = integer()

	      Returns the current total and accept intensities, the configuration parameters, and absolute counts of the total number of requests,
	      and accepted number of requests (since the overload process was started).

SEE ALSO

       alarm_handler(3erl), sasl(3erl)

Ericsson AB							   sasl 2.1.9.3 						    overload(3erl)