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<H1>overload</H1>
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<H3>MODULE</H3>
<DIV CLASS=REFBODY>
overload
</DIV>

<H3>MODULE SUMMARY</H3>
<DIV CLASS=REFBODY>
An Overload Regulation Process
</DIV>

<H3>DESCRIPTION</H3>
<DIV CLASS=REFBODY>

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


<P><CODE>overload</CODE> is part of the <CODE>sasl</CODE> application, and all
configuration parameters are defined there.


<P>A set of two intensities are maintained, the <CODE>total
intensity</CODE> and the <CODE>accept intensity</CODE>. For that purpose
there are two configuration parameters, the <CODE>MaxIntensity</CODE>
and the <CODE>Weight</CODE> value (both are measured in 1/second).


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


<P>
<UL>

<LI>
The current <CODE>total intensity</CODE>, denoted
         <CODE>TI(n)</CODE>, is calculated according to the formula,
         
        <BR>
<CODE>TI(n) = exp(-Weight*(T(n) - T(n-1)) * TI(n-1) + Weight</CODE>,
         
        <BR>
where <CODE>TI(n-1)</CODE> is the previous total intensity.
         
<BR>

</LI>


<LI>
The current <CODE>accept intensity</CODE>, denoted
        <CODE>AI(n)</CODE>, is determined by the formula,
         
        <BR>
<CODE>AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1) + Weight</CODE>,
         
        <BR>
where <CODE>AI(n-1)</CODE> is the previous accept intensity,
         provided that the value of <CODE>exp(-Weight*(T(n) - T(n-1)) *
         AI(n-1)</CODE> is less than <CODE>MaxIntensity</CODE>; otherwise the
         value is
         
        <BR>
<CODE>AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1)</CODE>.
         
<BR>

</LI>


</UL>

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


<P><CODE>T = 1/Weight</CODE> 
<P>can be thought of as the &#34;time constant&#34;
of the intensity calculation formulas. For example, if <CODE>Weight =
0.1</CODE>, then a change in the underlying input intensity will be
reflected in the <CODE>total</CODE> and <CODE>accept intensities</CODE> within
approximately 10 seconds.


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


<P>
<DL>

<DT>
<CODE>{overload, []}</CODE>
</DT>

<DD>
This alarm is set when the current accept intensity exceeds
         <CODE>MaxIntensity</CODE>. 
         
<BR>

</DD>

</DL>

<P>A new overload alarm is not set until the current accept
intensity has fallen below <CODE>MaxIntensity</CODE>. 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 <CODE>MaxIntensity</CODE>, and it is not until then that
the alarm can be set again.



</DIV>

<H3>EXPORTS</H3>

<P><A NAME="request/0"><STRONG><CODE>request() -&#62; accept | reject</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY>

<P>Returns <CODE>accept</CODE> or <CODE>reject</CODE> depending on the
         current value of the accept intensity. 
<P>The application
         calling this function should be processed with the job in
         question if the return value is <CODE>accept</CODE>; otherwise it
         should not continue with that job.
         

</DIV>

<P><A NAME="get_overload_info/0"><STRONG><CODE>get_overload_info() -&#62; OverloadInfo</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>OverloadInfo = [{total_intensity, TotalIntensity},
         {accept_intensity, AcceptIntensity}, {max_intensity,
         MaxIntensity}, {weight, Weight}, {total_requests,
         TotalRequests}, {accepted_requests, AcceptedRequests}].</CODE></STRONG><BR>
<STRONG><CODE>TotalIntensity = float() &#62; 0</CODE></STRONG><BR>
<STRONG><CODE>AcceptIntensity = float() &#62; 0</CODE></STRONG><BR>
<STRONG><CODE>MaxIntensity = float() &#62; 0</CODE></STRONG><BR>
<STRONG><CODE>Weight = float() &#62; 0</CODE></STRONG><BR>
<STRONG><CODE>TotalRequests = integer()</CODE></STRONG><BR>
<STRONG><CODE>AcceptedRequests = integer()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>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).      

</DIV>

<H3>See Also</H3>
<DIV CLASS=REFBODY>

<P> alarm_handler(3), sasl(3)



</DIV>

<H3>AUTHORS</H3>
<DIV CLASS=REFBODY>
Martin Bjrklund - support@erlang.ericsson.se<BR>
Peter Hgfeldt - support@erlang.ericsson.se<BR>

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<SMALL>sasl 2.0<BR>
Copyright &copy; 1991-2004
<A HREF="http://www.erlang.se">Ericsson AB</A><BR>
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