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<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>overload</title>
    <prepared>Peter H&ouml;gfeldt</prepared>
    <responsible>Peter H&ouml;gfeldt</responsible>
    <docno></docno>
    <approved>(Joe Armstrong)</approved>
    <checked></checked>
    <date>1996-10-29</date>
    <rev>A</rev>
    <file>overload.sgml</file>
  </header>
  <module>overload</module>
  <modulesummary>An Overload Regulation Process</modulesummary>
  <description>
    <p><c>overload</c> is a process which indirectly regulates CPU
      usage in the system. The idea is that a main application calls
      the <c>request/0</c> 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>
    <p><c>overload</c> is part of the <c>sasl</c> application, and all
      configuration parameters are defined there.
      </p>
    <p>A set of two intensities are maintained, the <c>total intensity</c> and the <c>accept intensity</c>. For that purpose
      there are two configuration parameters, the <c>MaxIntensity</c>
      and the <c>Weight</c> value (both are measured in 1/second).
      </p>
    <p>Then total and accept intensities are calculated as
      follows. Assume that the time of the current call to
      <c>request/0</c> is <c>T(n)</c>, and that the time of the
      previous call was <c>T(n-1)</c>.
      </p>
    <list type="bulleted">
      <item>
        <p>The current <c>total intensity</c>, denoted
          <c>TI(n)</c>, is calculated according to the formula,
          </p>
        <p><c>TI(n) = exp(-Weight*(T(n) - T(n-1)) * TI(n-1) +  Weight</c>,
          </p>
        <p>where <c>TI(n-1)</c> is the previous total intensity.
          </p>
      </item>
      <item>
        <p>The current <c>accept intensity</c>, denoted
          <c>AI(n)</c>, is determined by the formula,
          </p>
        <p><c>AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1) + Weight</c>,
          </p>
        <p>where <c>AI(n-1)</c> is the previous accept intensity,
          provided that the value of <c>exp(-Weight*(T(n) - T(n-1)) * AI(n-1)</c> is less than <c>MaxIntensity</c>; otherwise the
          value is
          </p>
        <p><c>AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1)</c>.
          </p>
      </item>
    </list>
    <p>The value of configuration parameter <c>Weight</c> controls the
      speed with which the calculations of intensities will react to
      changes in the underlying input intensity. The inverted value of
      <c>Weight</c>,
      </p>
    <p><c>T = 1/Weight</c></p>
    <p>can be thought of as the "time constant"
      of the intensity calculation formulas. For example, if <c>Weight = 0.1</c>, then a change in the underlying input intensity will be
      reflected in the <c>total</c> and <c>accept intensities</c> within
      approximately 10 seconds.
      </p>
    <p>The overload process defines one alarm, which it sets using
      <c>alarm_handler:set_alarm(Alarm)</c>.  <c>Alarm</c> is defined
      as:
      </p>
    <taglist>
      <tag><c>{overload, []}</c></tag>
      <item>
        <p>This alarm is set when the current accept intensity exceeds
          <c>MaxIntensity</c>. 
          </p>
      </item>
    </taglist>
    <p>A new overload alarm is not set until the current accept
      intensity has fallen below <c>MaxIntensity</c>. 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 <c>MaxIntensity</c>, and it is not until then that
      the alarm can be set again.
      </p>
  </description>
  <funcs>
    <func>
      <name>request() -> accept | reject</name>
      <fsummary>Request to proceed with current job</fsummary>
      <desc>
        <p>Returns <c>accept</c> or <c>reject</c> depending on the
          current value of the accept intensity.  </p>
        <p>The application
          calling this function should be processed with the job in
          question if the return value is <c>accept</c>; otherwise it
          should not continue with that job.
          </p>
      </desc>
    </func>
    <func>
      <name>get_overload_info() -> OverloadInfo</name>
      <fsummary>Return current overload information data</fsummary>
      <type>
        <v>OverloadInfo = [{total_intensity, TotalIntensity}, {accept_intensity, AcceptIntensity}, {max_intensity, MaxIntensity}, {weight, Weight}, {total_requests, TotalRequests}, {accepted_requests, AcceptedRequests}].</v>
        <v>TotalIntensity = float() > 0</v>
        <v>AcceptIntensity = float() > 0</v>
        <v>MaxIntensity = float() > 0</v>
        <v>Weight = float() > 0</v>
        <v>TotalRequests = integer()</v>
        <v>AcceptedRequests = integer()</v>
      </type>
      <desc>
        <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).</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p>alarm_handler(3), sasl(3)
      </p>
  </section>
</erlref>