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<!-- refpage --><center><h1>overload</h1></center>
<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>
<p><span class="code">overload</span> is a process which indirectly regulates CPU
usage in the system. The idea is that a main application calls
the <span class="code">request/0</span> 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><span class="code">overload</span> is part of the <span class="code">sasl</span> application, and all
configuration parameters are defined there.
</p>
<p>A set of two intensities are maintained, the <span class="code">total intensity</span> and the <span class="code">accept intensity</span>. For that purpose
there are two configuration parameters, the <span class="code">MaxIntensity</span>
and the <span class="code">Weight</span> 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
<span class="code">request/0</span> is <span class="code">T(n)</span>, and that the time of the
previous call was <span class="code">T(n-1)</span>.
</p>
<ul>
<li>
<p>The current <span class="code">total intensity</span>, denoted
<span class="code">TI(n)</span>, is calculated according to the formula,
</p>
<p><span class="code">TI(n) = exp(-Weight*(T(n) - T(n-1)) * TI(n-1) + Weight</span>,
</p>
<p>where <span class="code">TI(n-1)</span> is the previous total intensity.
</p>
</li>
<li>
<p>The current <span class="code">accept intensity</span>, denoted
<span class="code">AI(n)</span>, is determined by the formula,
</p>
<p><span class="code">AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1) + Weight</span>,
</p>
<p>where <span class="code">AI(n-1)</span> is the previous accept intensity,
provided that the value of <span class="code">exp(-Weight*(T(n) - T(n-1)) * AI(n-1)</span> is less than <span class="code">MaxIntensity</span>; otherwise the
value is
</p>
<p><span class="code">AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1)</span>.
</p>
</li>
</ul>
<p>The value of configuration parameter <span class="code">Weight</span> controls the
speed with which the calculations of intensities will react to
changes in the underlying input intensity. The inverted value of
<span class="code">Weight</span>,
</p>
<p><span class="code">T = 1/Weight</span></p>
<p>can be thought of as the "time constant"
of the intensity calculation formulas. For example, if <span class="code">Weight = 0.1</span>, then a change in the underlying input intensity will be
reflected in the <span class="code">total</span> and <span class="code">accept intensities</span> within
approximately 10 seconds.
</p>
<p>The overload process defines one alarm, which it sets using
<span class="code">alarm_handler:set_alarm(Alarm)</span>. <span class="code">Alarm</span> is defined
as:
</p>
<dl>
<dt><strong><span class="code">{overload, []}</span></strong></dt>
<dd>
<p>This alarm is set when the current accept intensity exceeds
<span class="code">MaxIntensity</span>.
</p>
</dd>
</dl>
<p>A new overload alarm is not set until the current accept
intensity has fallen below <span class="code">MaxIntensity</span>. 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 <span class="code">MaxIntensity</span>, and it is not until then that
the alarm can be set again.
</p>
</p></div>
<h3>EXPORTS</h3>
<p><a name="request-0"><span class="bold_code">request() -> accept | reject</span></a><br></p>
<div class="REFBODY"><p>
<p>Returns <span class="code">accept</span> or <span class="code">reject</span> 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 <span class="code">accept</span>; otherwise it
should not continue with that job.
</p>
</p></div>
<p><a name="get_overload_info-0"><span class="bold_code">get_overload_info() -> OverloadInfo</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">OverloadInfo = [{total_intensity, TotalIntensity}, {accept_intensity, AcceptIntensity}, {max_intensity, MaxIntensity}, {weight, Weight}, {total_requests, TotalRequests}, {accepted_requests, AcceptedRequests}].</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">TotalIntensity = float() > 0</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">AcceptIntensity = float() > 0</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">MaxIntensity = float() > 0</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Weight = float() > 0</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">TotalRequests = integer()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">AcceptedRequests = integer()</span><br>
</div>
</div>
<div class="REFBODY"><p>
<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>
</p></div>
<h3><a name="id64890">See Also</a></h3>
<div class="REFBODY">
<p>alarm_handler(3), sasl(3)
</p>
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