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<h1 class="topictitle1">Timing</h1>
<div><p>When measuring the performance of parallel programs, it is usually <em>wall clock</em> time, not CPU time, that matters. The reason is that better parallelization typically increases aggregate CPU time by employing more CPUs. The goal of parallelizing a program is usually to make it run <em>faster</em> in real time.</p>
<p>The class <samp class="codeph">tick_count</samp> in Intel® Threading Building Blocks (Intel® TBB) provides a simple interface for measuring wall clock time. A <samp class="codeph">tick_count</samp> value obtained from the static method <span class="option">tick_count::now()</span> represents the current absolute time. Subtracting two <samp class="codeph">tick_count</samp> values yields a relative time in <samp class="codeph">tick_count::interval_t</samp>, which you can convert to seconds, as in the following example:</p>
<pre>tick_count t0 = tick_count::now();
... do some work ...
tick_count t1 = tick_count::now();
printf("work took %g seconds\n",(t1-t0).seconds());</pre><p>Unlike some timing interfaces, <samp class="codeph">tick_count</samp> is guaranteed to be safe to use across threads. It is valid to subtract <samp class="codeph">tick_count</samp> values that were created by different threads. A <samp class="codeph">tick_count</samp> difference can be converted to seconds.</p>
<p>The resolution of <samp class="codeph">tick_count</samp> corresponds to the highest resolution timing service on the platform that is valid across threads in the same process. Since the CPU timer registers are <em>not</em> valid across threads on some platforms, this means that the resolution of <span class="option">tick_count</span> can not be guaranteed to be consistent across platforms.</p>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="../tbb_userguide/title.htm">Intel® Threading Building Blocks (Intel® TBB) User Guide</a></div>
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