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<html><head><title>Threads (Programming Perl)</title><!-- STYLESHEET --><link rel="stylesheet" type="text/css" href="../style/style1.css"><!-- METADATA --><!--Dublin Core Metadata--><meta name="DC.Creator" content=""><meta name="DC.Date" content=""><meta name="DC.Format" content="text/xml" scheme="MIME"><meta name="DC.Generator" content="XSLT stylesheet, xt by James Clark"><meta name="DC.Identifier" content=""><meta name="DC.Language" content="en-US"><meta name="DC.Publisher" content="O'Reilly & Associates, Inc."><meta name="DC.Source" content="" scheme="ISBN"><meta name="DC.Subject.Keyword" content=""><meta name="DC.Title" content="Threads"><meta name="DC.Type" content="Text.Monograph"></head><body><!-- START OF BODY --><!-- TOP BANNER --><img src="gifs/smbanner.gif" usemap="#banner-map" border="0" alt="Book Home"><map name="banner-map"><AREA SHAPE="RECT" COORDS="0,0,466,71" HREF="index.htm" ALT="Programming Perl"><AREA SHAPE="RECT" COORDS="467,0,514,18" HREF="jobjects/fsearch.htm" ALT="Search this book"></map><!-- TOP NAV BAR --><div class="navbar"><table width="515" border="0"><tr><td align="left" valign="top" width="172"><a href="ch16_05.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="part3.htm">Part 3: Perl as Technology</a></td><td align="right" valign="top" width="172"><a href="ch17_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr></table></div><hr width="515" align="left"><!-- SECTION BODY --><h1 class="chapter">Chapter 17. Threads</h1><div class="htmltoc"><h4 class="tochead">Contents:</h4><p><a href="ch17_01.htm">The Process Model</a><br><a href="ch17_02.htm">The Thread Model</a><br></p></div><p><a name="INDEX-3119"></a>Parallel programming is much harder than it looks. Imagine taking arecipe from a cookbook and converting it into something that severaldozen chefs can work on all at the same time. You can take twoapproaches.</p><p>One approach is to give each chef a private kitchen, completewith its own supply of raw materials and utensils. For recipes that can bedivided up into parts easily, and for foods that can be transportedfrom kitchen to kitchen easily, this approach works well because itkeeps the chefs out of each other's kitchens.</p><p>Alternatively, you can just put all the chefs into one kitchen, and let themwork things out, like who gets to use the mixer when. This can get messy,especially when the meat cleavers start to fly.</p><p><a name="INDEX-3120"></a>These two approaches correspond to two models of parallel programmingon computers. The first is the multiprocessing model typical oftraditional Unix systems, in which each thread of control has its ownset of resources, which taken together we call a process. The secondmodel is the multithreading model, in which each thread of controlshares resources with all other threads of control. Or doesn't share,as the case may be (and upon occasion must be).</p><p>We all know that chefs like to be in control; that's okay, because chefs<em class="emphasis">need</em> to be in control in order to accomplish what we want them toaccomplish. But chefs need to be organized, one way or another.</p><p>Perl supports both models of organization. In this chapter we'llcall them the <em class="emphasis">process model</em> and the <em class="emphasis">thread model</em>.</p><h2 class="sect1">17.1. The Process Model</h2><p><a name="INDEX-3121"></a><a name="INDEX-3122"></a><a name="INDEX-3123"></a>We'll not discuss the process model in great detail here, simplybecause it's pervasive throughout the rest of this book. Perloriginated on Unix systems, so it is steeped in the notion that eachprocess does its own thing. If a process wants to start some parallelprocessing, then logically it has to start a parallel process; that is,it must fork a new heavyweight process, which by default shares littlewith the parent process except some file descriptors. (It may seemlike parent and child are sharing a lot more, but most of the state ofthe parent process is merely duplicated in the child process and notreally shared in a logical sense. The operating system may of courseexhibit laziness in enforcing that logical separation, in which case wecall it copy-on-write semantics, but we wouldn't be doing the copy atall unless there were a logical separation first.)<a name="INDEX-3124"></a></p><p>Historically, this industrial-strength view of multiprocessing hasposed a bit of a problem on Microsoft systems, because Windows has nothad a well-developed multiprocessing model (and what it does have inthat regard, it doesn't often rely on for parallel programming). It hastypically taken a multithreading approach instead.<a name="INDEX-3125"></a></p><p><a name="INDEX-3126"></a>However, through heroic efforts, version 5.6 of Perl now implements the<tt class="literal">fork</tt> operation on Windows by cloning a new interpreter object withinthe same process. That means that most examples using <tt class="literal">fork</tt> in therest of the book will now work on Windows. The cloned interpretershares immutable code with other interpreters but gets its own copy ofdata to play with. (There can still be problems with C libraries thatdon't understand threads, of course.)</p><p><a name="INDEX-3127"></a><a name="INDEX-3128"></a>This approach to multiprocessing has been christened <em class="emphasis">ithreads</em>, shortfor "interpreter threads". The initial impetus for implementingithreads was to emulate <tt class="literal">fork</tt> for Microsoft systems. However, we quickly realized that, although the other interpreters arerunning as distinct threads, they're running in the same process,so it would be easy to make these separate interpreters share data,even though they don't share by default.</p><p>This is the opposite of the typical threading model, in which everythingis shared by default, and you have to take pains <em class="emphasis">not</em> to share something.But you should not view these two models as totally distinct from eachother, because they are both trying to bridge the same river;they're just building from opposite shores. The actual solutionto any parallel processing problem is going to involve some degree ofsharing, together with some degree of selfishness.</p><p>So over the long run, the intent is to extend the ithreads model toallow as much sharing as you need or want. However, as of thiswriting, the only user-visible interface for ithreads is the<tt class="literal">fork</tt> call under Microsoft ports of Perl. We thinkthat, eventually, this approach will produce cleaner programs than thestandard threading approach. Basically, it's easier to run an economywhere you assume everyone owns what they own, rather than assumingthat everyone owns everything. Not that people aren't expected toshare in a capitalist economy, or peculate<a href="#FOOTNOTE-1">[1]</a> in a communist economy.These things tend toward the middle. Socialism happens. But withlarge groups of people, sharing everything by default only works whenyou have a "head chef" with a big meat cleaver who thinks heowns everything.</p><blockquote class="footnote"><a name="FOOTNOTE-1"></a><p>[1] peculate:<em class="emphasis">v.i.</em>, to swipe the People's Property from thecommons in the middle of the night; to embezzle from the publicsomething that is not necessarily money(~L. <em class="emphasis">peculiar</em>, "not common"), cf<em class="emphasis">embrace</em>, <em class="emphasis">extend</em>,<em class="emphasis">GPL</em>.</p></blockquote><p>Of course, the actual government of any computer is run by that fascistdictator known as the operating system. But a wise dictator knows whento let the people think they're capitalists--and when to let them thinkthey're communists.</p><a name="INDEX-3219"></a><!-- BOTTOM NAV BAR --><hr width="515" align="left"><div class="navbar"><table width="515" border="0"><tr><td align="left" valign="top" width="172"><a href="ch16_05.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0"></a></td><td align="right" valign="top" width="172"><a href="ch17_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr><tr><td align="left" valign="top" width="172">16.5. Sockets</td><td align="center" valign="top" width="171"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0"></a></td><td align="right" valign="top" width="172">17.2. The Thread Model</td></tr></table></div><hr width="515" align="left"><!-- LIBRARY NAV BAR --><img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p><font size="-1"><a href="copyrght.htm">Copyright © 2001</a> O'Reilly & Associates. All rights reserved.</font></p><map name="library-map"> <area shape="rect" coords="2,-1,79,99" href="../index.htm"><area shape="rect" coords="84,1,157,108" href="../perlnut/index.htm"><area shape="rect" coords="162,2,248,125" href="../prog/index.htm"><area shape="rect" coords="253,2,326,130" href="../advprog/index.htm"><area shape="rect" coords="332,1,407,112" href="../cookbook/index.htm"><area shape="rect" coords="414,2,523,103" href="../sysadmin/index.htm"></map><!-- END OF BODY --></body></html>⌨️ 快捷键说明
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