<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Low-level Thread Support — Python Module of the Week</title> <link rel="stylesheet" href="../_static/sphinxdoc.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '1.132', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <link rel="author" title="About these documents" href="../about.html" /> <link rel="top" title="Python Module of the Week" href="../index.html" /> <link rel="up" title="sys – System-specific Configuration" href="index.html" /> <link rel="next" title="Modules and Imports" href="imports.html" /> <link rel="prev" title="Tracing a Program As It Runs" href="tracing.html" /> </head> <body> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="imports.html" title="Modules and Imports" accesskey="N">next</a> |</li> <li class="right" > <a href="tracing.html" title="Tracing a Program As It Runs" accesskey="P">previous</a> |</li> <li><a href="../contents.html">PyMOTW</a> »</li> <li><a href="../runtime_services.html" >Python Runtime Services</a> »</li> <li><a href="index.html" accesskey="U">sys – System-specific Configuration</a> »</li> </ul> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Low-level Thread Support</a><ul> <li><a class="reference internal" href="#check-interval">Check Interval</a></li> <li><a class="reference internal" href="#debugging">Debugging</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="tracing.html" title="previous chapter">Tracing a Program As It Runs</a></p> <h4>Next topic</h4> <p class="topless"><a href="imports.html" title="next chapter">Modules and Imports</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/sys/threads.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="low-level-thread-support"> <span id="sys-threads"></span><h1>Low-level Thread Support<a class="headerlink" href="#low-level-thread-support" title="Permalink to this headline">¶</a></h1> <p><a class="reference internal" href="index.html#module-sys" title="sys: System-specific configuration"><tt class="xref py py-mod docutils literal"><span class="pre">sys</span></tt></a> includes low-level functions for controlling and debugging thread behavior.</p> <div class="section" id="check-interval"> <h2>Check Interval<a class="headerlink" href="#check-interval" title="Permalink to this headline">¶</a></h2> <p>Python 2 uses a form of cooperative multitasking in its thread implementation. At a fixed interval, bytecode execution is paused and the interpreter checks if any signal handlers need to be executed. During the same interval check, the global interpreter lock is also released by the current thread and then reacquired, giving other threads an opportunity to take over execution by grabbing the lock first.</p> <p>The default check interval is 100 bytecodes and the current value can always be retrieved with <tt class="xref py py-func docutils literal"><span class="pre">sys.getcheckinterval()</span></tt>. Changing the interval with <tt class="xref py py-func docutils literal"><span class="pre">sys.setcheckinterval()</span></tt> may have an impact on the performance of an application, depending on the nature of the operations being performed.</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span> <span class="kn">import</span> <span class="nn">threading</span> <span class="kn">from</span> <span class="nn">Queue</span> <span class="kn">import</span> <span class="n">Queue</span> <span class="kn">import</span> <span class="nn">time</span> <span class="k">def</span> <span class="nf">show_thread</span><span class="p">(</span><span class="n">q</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="p">):</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span> <span class="k">for</span> <span class="n">j</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">extraByteCodes</span><span class="p">):</span> <span class="k">pass</span> <span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">threading</span><span class="o">.</span><span class="n">current_thread</span><span class="p">()</span><span class="o">.</span><span class="n">name</span><span class="p">)</span> <span class="k">return</span> <span class="k">def</span> <span class="nf">run_threads</span><span class="p">(</span><span class="n">prefix</span><span class="p">,</span> <span class="n">interval</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="p">):</span> <span class="k">print</span> <span class="s">'</span><span class="si">%(prefix)s</span><span class="s"> interval = </span><span class="si">%(interval)s</span><span class="s"> with </span><span class="si">%(extraByteCodes)s</span><span class="s"> extra operations'</span> <span class="o">%</span> <span class="nb">locals</span><span class="p">()</span> <span class="n">sys</span><span class="o">.</span><span class="n">setcheckinterval</span><span class="p">(</span><span class="n">interval</span><span class="p">)</span> <span class="n">q</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span> <span class="n">threads</span> <span class="o">=</span> <span class="p">[</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">show_thread</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s">'</span><span class="si">%s</span><span class="s"> T</span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">prefix</span><span class="p">,</span> <span class="n">i</span><span class="p">),</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">q</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="p">)</span> <span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span> <span class="p">]</span> <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span> <span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span> <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span> <span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span> <span class="k">while</span> <span class="ow">not</span> <span class="n">q</span><span class="o">.</span><span class="n">empty</span><span class="p">():</span> <span class="k">print</span> <span class="n">q</span><span class="o">.</span><span class="n">get</span><span class="p">()</span> <span class="k">print</span> <span class="k">return</span> <span class="n">run_threads</span><span class="p">(</span><span class="s">'Default'</span><span class="p">,</span> <span class="n">interval</span><span class="o">=</span><span class="mi">10</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="o">=</span><span class="mi">1000</span><span class="p">)</span> <span class="n">run_threads</span><span class="p">(</span><span class="s">'Custom'</span><span class="p">,</span> <span class="n">interval</span><span class="o">=</span><span class="mi">10</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span> </pre></div> </div> <p>When the check interval is smaller than the number of bytecodes in a thread, the interpreter may give another thread control so that it runs for a while. This is illustrated in the first set of output where the check interval is set to 100 (the default) and 1000 extra loop iterations are performed for each step through the <tt class="docutils literal"><span class="pre">i</span></tt> loop.</p> <p>On the other hand, when the check interval is <em>greater</em> than the number of bytecodes being executed by a thread that doesn’t release control for another reason, the thread will finish its work before the interval comes up. This is illustrated by the order of the name values in the queue in the second example.</p> <div class="highlight-python"><pre>$ python sys_checkinterval.py Default interval = 10 with 1000 extra operations Default T0 Default T0 Default T0 Default T1 Default T2 Default T2 Default T0 Default T1 Default T2 Default T0 Default T1 Default T2 Default T1 Default T2 Default T1 Custom interval = 10 with 0 extra operations Custom T0 Custom T0 Custom T0 Custom T0 Custom T0 Custom T1 Custom T1 Custom T1 Custom T1 Custom T1 Custom T2 Custom T2 Custom T2 Custom T2 Custom T2</pre> </div> <p>Modifying the check interval is not as clearly useful as it might seem. Many other factors may control the context switching behavior of Python’s threads. For example, if a thread performs I/O, it releases the GIL and may therefore allow another thread to take over execution.</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span> <span class="kn">import</span> <span class="nn">threading</span> <span class="kn">from</span> <span class="nn">Queue</span> <span class="kn">import</span> <span class="n">Queue</span> <span class="kn">import</span> <span class="nn">time</span> <span class="k">def</span> <span class="nf">show_thread</span><span class="p">(</span><span class="n">q</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="p">):</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span> <span class="k">for</span> <span class="n">j</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">extraByteCodes</span><span class="p">):</span> <span class="k">pass</span> <span class="c">#q.put(threading.current_thread().name)</span> <span class="k">print</span> <span class="n">threading</span><span class="o">.</span><span class="n">current_thread</span><span class="p">()</span><span class="o">.</span><span class="n">name</span> <span class="k">return</span> <span class="k">def</span> <span class="nf">run_threads</span><span class="p">(</span><span class="n">prefix</span><span class="p">,</span> <span class="n">interval</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="p">):</span> <span class="k">print</span> <span class="s">'</span><span class="si">%(prefix)s</span><span class="s"> interval = </span><span class="si">%(interval)s</span><span class="s"> with </span><span class="si">%(extraByteCodes)s</span><span class="s"> extra operations'</span> <span class="o">%</span> <span class="nb">locals</span><span class="p">()</span> <span class="n">sys</span><span class="o">.</span><span class="n">setcheckinterval</span><span class="p">(</span><span class="n">interval</span><span class="p">)</span> <span class="n">q</span> <span class="o">=</span> <span class="n">Queue</span><span class="p">()</span> <span class="n">threads</span> <span class="o">=</span> <span class="p">[</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">show_thread</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s">'</span><span class="si">%s</span><span class="s"> T</span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">prefix</span><span class="p">,</span> <span class="n">i</span><span class="p">),</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">q</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="p">)</span> <span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span> <span class="p">]</span> <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span> <span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span> <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span> <span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span> <span class="k">while</span> <span class="ow">not</span> <span class="n">q</span><span class="o">.</span><span class="n">empty</span><span class="p">():</span> <span class="k">print</span> <span class="n">q</span><span class="o">.</span><span class="n">get</span><span class="p">()</span> <span class="k">print</span> <span class="k">return</span> <span class="n">run_threads</span><span class="p">(</span><span class="s">'Default'</span><span class="p">,</span> <span class="n">interval</span><span class="o">=</span><span class="mi">100</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="o">=</span><span class="mi">1000</span><span class="p">)</span> <span class="n">run_threads</span><span class="p">(</span><span class="s">'Custom'</span><span class="p">,</span> <span class="n">interval</span><span class="o">=</span><span class="mi">10</span><span class="p">,</span> <span class="n">extraByteCodes</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span> </pre></div> </div> <p>This example is modified from the first so that the thread prints directly to <tt class="xref py py-const docutils literal"><span class="pre">sys.stdout</span></tt> instead of appending to a queue. The output is much less predictable.</p> <div class="highlight-python"><pre>$ python sys_checkinterval_io.py Default interval = 100 with 1000 extra operations Default T0 Default T1 Default T1Default T2 Default T0Default T2 Default T2 Default T2 Default T1 Default T2 Default T1 Default T1 Default T0 Default T0 Default T0 Custom interval = 10 with 0 extra operations Custom T0 Custom T0 Custom T0 Custom T0 Custom T0 Custom T1 Custom T1 Custom T1 Custom T1 Custom T2 Custom T2 Custom T2 Custom T1Custom T2 Custom T2</pre> </div> <div class="admonition-see-also admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><a class="reference internal" href="../dis/index.html#module-dis" title="dis: Python Bytecode Disassembler"><tt class="xref py py-mod docutils literal"><span class="pre">dis</span></tt></a></dt> <dd>Disassembling your Python code with the dis module is one way to count bytecodes.</dd> </dl> </div> </div> <div class="section" id="debugging"> <h2>Debugging<a class="headerlink" href="#debugging" title="Permalink to this headline">¶</a></h2> <p>Identifying deadlocks can be on of the most difficult aspects of working with threads. <tt class="xref py py-func docutils literal"><span class="pre">sys._current_frames()</span></tt> can help by showing exactly where a thread is stopped.</p> <div class="highlight-python"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44</pre></div></td><td class="code"><div class="highlight"><pre><span class="c">#!/usr/bin/env python</span> <span class="c"># encoding: utf-8</span> <span class="kn">import</span> <span class="nn">sys</span> <span class="kn">import</span> <span class="nn">threading</span> <span class="kn">import</span> <span class="nn">time</span> <span class="n">io_lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span> <span class="n">blocker</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span> <span class="k">def</span> <span class="nf">block</span><span class="p">(</span><span class="n">i</span><span class="p">):</span> <span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">current_thread</span><span class="p">()</span> <span class="k">with</span> <span class="n">io_lock</span><span class="p">:</span> <span class="k">print</span> <span class="s">'</span><span class="si">%s</span><span class="s"> with ident </span><span class="si">%s</span><span class="s"> going to sleep'</span> <span class="o">%</span> <span class="p">(</span><span class="n">t</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">t</span><span class="o">.</span><span class="n">ident</span><span class="p">)</span> <span class="k">if</span> <span class="n">i</span><span class="p">:</span> <span class="n">blocker</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span> <span class="c"># acquired but never released</span> <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.2</span><span class="p">)</span> <span class="k">with</span> <span class="n">io_lock</span><span class="p">:</span> <span class="k">print</span> <span class="n">t</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="s">'finishing'</span> <span class="k">return</span> <span class="c"># Create and start several threads that "block"</span> <span class="n">threads</span> <span class="o">=</span> <span class="p">[</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">block</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">i</span><span class="p">,))</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span> <span class="p">]</span> <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span> <span class="n">t</span><span class="o">.</span><span class="n">setDaemon</span><span class="p">(</span><span class="bp">True</span><span class="p">)</span> <span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span> <span class="c"># Map the threads from their identifier to the thread object</span> <span class="n">threads_by_ident</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">((</span><span class="n">t</span><span class="o">.</span><span class="n">ident</span><span class="p">,</span> <span class="n">t</span><span class="p">)</span> <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">)</span> <span class="c"># Show where each thread is "blocked"</span> <span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.01</span><span class="p">)</span> <span class="k">with</span> <span class="n">io_lock</span><span class="p">:</span> <span class="k">for</span> <span class="n">ident</span><span class="p">,</span> <span class="n">frame</span> <span class="ow">in</span> <span class="n">sys</span><span class="o">.</span><span class="n">_current_frames</span><span class="p">()</span><span class="o">.</span><span class="n">items</span><span class="p">():</span> <span class="n">t</span> <span class="o">=</span> <span class="n">threads_by_ident</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">ident</span><span class="p">)</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">t</span><span class="p">:</span> <span class="c"># Main thread</span> <span class="k">continue</span> <span class="k">print</span> <span class="n">t</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="s">'stopped in'</span><span class="p">,</span> <span class="n">frame</span><span class="o">.</span><span class="n">f_code</span><span class="o">.</span><span class="n">co_name</span><span class="p">,</span> <span class="k">print</span> <span class="s">'at line'</span><span class="p">,</span> <span class="n">frame</span><span class="o">.</span><span class="n">f_lineno</span><span class="p">,</span> <span class="s">'of'</span><span class="p">,</span> <span class="n">frame</span><span class="o">.</span><span class="n">f_code</span><span class="o">.</span><span class="n">co_filename</span> <span class="c"># Let the threads finish</span> <span class="c"># for t in threads:</span> <span class="c"># t.join()</span> </pre></div> </td></tr></table></div> <p>The dictionary returned by <tt class="xref py py-func docutils literal"><span class="pre">sys._current_frames()</span></tt> is keyed on the thread identifier, rather than its name. A little work is needed to map those identifiers back to the thread object.</p> <p>Since <strong>Thread-1</strong> does not sleep, it finishes before its status is checked. Since it is no longer active, it does not appear in the output. <strong>Thread-2</strong> acquires the lock <em>blocker</em>, then sleeps for a short period. Meanwhile <strong>Thread-3</strong> tries to acquire <em>blocker</em> but cannot because <strong>Thread-2</strong> already has it.</p> <div class="highlight-python"><pre>$ python sys_current_frames.py Thread-1 with ident 4300550144 going to sleep Thread-1 finishing Thread-2 with ident 4301086720 going to sleep Thread-3 with ident 4302835712 going to sleep Thread-3 stopped in block at line 16 of sys_current_frames.py Thread-2 stopped in block at line 17 of sys_current_frames.py</pre> </div> <div class="admonition-see-also admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><a class="reference internal" href="../threading/index.html#module-threading" title="threading: Manage several concurrent threads of execution."><tt class="xref py py-mod docutils literal"><span class="pre">threading</span></tt></a></dt> <dd>The threading module includes classes for creating Python threads.</dd> <dt><a class="reference internal" href="../Queue/index.html#module-Queue" title="Queue: Provides a thread-safe FIFO implementation"><tt class="xref py py-mod docutils literal"><span class="pre">Queue</span></tt></a></dt> <dd>The Queue module provides a thread-safe implementation of a FIFO data structure.</dd> <dt><a class="reference external" href="http://jessenoller.com/2009/02/01/python-threads-and-the-global-interpreter-lock/">Python Threads and the Global Interpreter Lock</a></dt> <dd>Jesse Noller’s article from the December 2007 issue of Python Magazine.</dd> <dt><a class="reference external" href="http://www.dabeaz.com/python/GIL.pdf">Inside the Python GIL</a></dt> <dd>Presentation by David Beazley describing thread implementation and performance issues, including how the check interval and GIL are related.</dd> </dl> </div> </div> </div> </div> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="imports.html" title="Modules and Imports" >next</a> |</li> <li class="right" > <a href="tracing.html" title="Tracing a Program As It Runs" >previous</a> |</li> <li><a href="../contents.html">PyMOTW</a> »</li> <li><a href="../runtime_services.html" >Python Runtime Services</a> »</li> <li><a href="index.html" >sys – System-specific Configuration</a> »</li> </ul> </div> <div class="footer"> © Copyright Doug Hellmann. 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