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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 parameters, 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 MaxIntensity; 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  underlying  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)