94 lines
4.1 KiB
HTML
94 lines
4.1 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>Thread priority and scheduling</title>
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<!-- Begin Header Records ========================================== -->
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<!-- NETMG2 SCRIPT A converted by B2H R4.1 (346) (CMS) by HOLTJM at -->
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<!-- RCHVMW2 on 29 Jan 1999 at 10:01:37 -->
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<!--File Edited November 2001 -->
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<!--End Header Records -->
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<link rel="stylesheet" type="text/css" href="../rzahg/ic.css">
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</head>
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<body>
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<!-- Java sync-link -->
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<script language="Javascript" src="../rzahg/synch.js" type="text/javascript">
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</script>
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<a name="Top_Of_Page"></a>
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<h2>Thread priority and scheduling</h2>
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<p>The default thread creation attributes of the iSeries implementation of
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Pthreads uses an explicitly specified priority of <strong>
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DEFAULT_PRIO_NP</strong>. Some implementations inherit the scheduling priority
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and policy of the creating thread by default. For better performance, the
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iSeries implementation chooses to start each thread with an explicit priority so
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that, when a thread is created, the priority of the creating thread does not
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need to be retrieved at run-time.</p>
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<p>An iSeries thread competes for scheduling resources against other threads in
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the system, not solely against other threads in the process. The scheduler is a
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delay cost scheduler based on several delay cost curves (priority ranges). The
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POSIX standard and the Single UNIX Specification refers to this as scheduling
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scope and scheduling policy, which cannot be changed from the default of
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<strong>SCHED_OTHER</strong> in this implementation.</p>
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<p>The following Pthread APIs support a scheduling policy of only
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SCHED_OTHER.</p>
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<ul>
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<li><strong>pthread_setschedparam</strong> (SCHED_OTHER only supported)</li>
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<li><strong>pthread_getschedparam</strong></li>
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<li><strong>pthread_attr_setschedparam</strong></li>
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<li><strong>pthread_attr_getschedparam</strong></li>
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</ul>
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<p>The priority of a thread is specified as a number that represents the value
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that is added to the priority of the process. Changing the priority of the
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process affects the priority of all of the threads within that process. The
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default priority for a thread is <strong>DEFAULT_PRIO_NP</strong>, which is no
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change from the process priority.</p>
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<p>On the iSeries, numerically lower priority values indicate higher priority with
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regard to scheduling. The <strong>pthread.h</strong> and <strong>
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sched.h</strong> header files define the priority constants in a way that is
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consistent with the threads standard, but opposite of priority specifications
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on the iSeries. When you specify a priority of -99 in a call to <strong>
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pthread_setschedparam</strong>(), the priority of the target thread is lowered
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to the lowest possible value.</p>
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<p>For example, process P1 is at iSeries priority 20 and contains a thread T1
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that specifies a Pthread priority adjustment of -18. Process P2 is at iSeries
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priority 25 and contains thread T2 that specifies a priority of -5. The result
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is that the system schedules the threads using the iSeries priority for T1 as 38
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and for T2 as 30. The thread scheduling is specified at a system level, and
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although process P2 runs at a lower priority ranking than process P1, thread T2
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within process P2 runs at a higher priority ranking than thread T1 in process
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P1, and thus gets more processing resources.</p>
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<hr>
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<center>
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<table cellpadding="2" cellspacing="2">
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<tr align="center">
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<td valign="middle" align="center">
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<a href="#Top_Of_Page">Top</a> |
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<a href="rzah4mst.htm">Pthread APIs</a> |
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<a href="aplist.htm">APIs by category</a></td>
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</tr>
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</table>
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</center>
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</body>
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</html>
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