aboutsummaryrefslogtreecommitdiffstats
path: root/lib/sasl/doc/src/overload.xml
diff options
context:
space:
mode:
authorErlang/OTP <[email protected]>2009-11-20 14:54:40 +0000
committerErlang/OTP <[email protected]>2009-11-20 14:54:40 +0000
commit84adefa331c4159d432d22840663c38f155cd4c1 (patch)
treebff9a9c66adda4df2106dfd0e5c053ab182a12bd /lib/sasl/doc/src/overload.xml
downloadotp-84adefa331c4159d432d22840663c38f155cd4c1.tar.gz
otp-84adefa331c4159d432d22840663c38f155cd4c1.tar.bz2
otp-84adefa331c4159d432d22840663c38f155cd4c1.zip
The R13B03 release.OTP_R13B03
Diffstat (limited to 'lib/sasl/doc/src/overload.xml')
-rw-r--r--lib/sasl/doc/src/overload.xml151
1 files changed, 151 insertions, 0 deletions
diff --git a/lib/sasl/doc/src/overload.xml b/lib/sasl/doc/src/overload.xml
new file mode 100644
index 0000000000..80457e75fa
--- /dev/null
+++ b/lib/sasl/doc/src/overload.xml
@@ -0,0 +1,151 @@
+<?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>
+