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<html><head><title>EXAMPLE 11: MEASURING TIMING JITTER</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./ex10_stack.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ex12_math.htm">[next]</a><h1>Example 11: Measuring Timing Jitter</h1><p>Real-time code should execute with more deterministic timing thannormal Linux process code. How much more deterministic is it? Thisexample measures the variation in the actual timing of a pure periodictask, using the Pentium's built-in Time Stamp Counter (TSC), and showsa plot something like this:<p><img src="./jitter.png"><p>Refer to the <ahref="../ex11_jitter/jitter_task.c">commented real-timesource code</a> and the <ahref="../ex11_jitter/jitter_app.c">commented applicationsource code</a> for the details.<h2>Principle of Operation</h2><ul><li>A pure periodic real-time task reads the TSC at the beginning ofeach 50-microsecond cycle. The TSC is available via the Intelinstruction RDTSC <a href="./references.htm#INT3B">[INT2B].</a> See <ahref="../ex11_jitter/tsc.c">tsc.c</a> for the code used here.</a><li>The TSC is a 64-bit unsigned number that increments each clockcycle. On a 1 GHz processor, this gives 1 nanosecond resolution. TheTSC will roll over after 2<sup>64</sup> cycles, which for the same 1GHz processor is over 584 years.<li>The TSC is logged into a shared memory array with 1K entries.<li>A Linux process waits for the array to fill up, and computes thedifference between successive entries. This should nominally be 50microseconds, but variation in the execution time due to the cache,branching in the scheduling algorith, etc. will introduce somevariation ("jitter"). <li>The differences are plotted to show the magnitude, which aretypically on the order of about 10 microseconds.<li>More detail on jitter analysis is available in <ahref="./references.htm#PRO">[PRO],</a> including a software techniquethat reduces jitter to below a tenth of a microsecond.</ul><h2>Running the Demo</h2>To run the demo, change to the 'ex11_jitter' subdirectory of thetop-level tutorial directory, and run the 'run' script by typing<pre>./run</pre>Alternatively, change to the top-level tutorial directory and run the'runall' script there by typing<pre>./runall</pre>and selecting the "Jitter Testing" button.<p>You'll see a plot window showing the magnitude of the jitter.<p><a href="../ex11_jitter/jitter_task.c">See the Real-Time Task Code</a><p><a href="../ex11_jitter/jitter_app.c">See the Linux Application Code</a><hr><a href="./ex12_math.htm">Next: Example 12, Floating Point in RealTime Tasks</a> <p><a href="./ex10_stack.htm">Back: Example 10, Determining Stack Size</a></body></html>⌨️ 快捷键说明
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