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POC sleep variance when ran by cron
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copied!<p>I wrote some C code to switch on and off some LEDs. Actually, I want to trigger them accurate to music but haven't found a better way than using <code>usleep()</code> in between yet.</p> <p>Anyway, turning on the LEDs, waiting at <code>usleep()</code> and turning off the LEDs again works pretty accurate when I call the program on the command line.</p> <p>Now I'd like cron to execute the program let's say every five minutes. Therefore I added following cronjob with <code>crontab -e</code>:</p> <pre><code>*/5 * * * * bash ~/startShow.sh >~/log 2>&1 </code></pre> <p>Same problem without the shell script.</p> <pre><code>*/5 * * * * ~/projects/startLEDShow >~/log 2>&1 </code></pre> <p>startShow.sh</p> <pre><code>date /usr/sbin/i2cset -y 1 0x40 0x00 0x21 # Preparation for communication via I2C cd projects ./startLEDShow </code></pre> <p>The program is triggered every five minutes, but at some point (not always the same point) the execution seems to stop for a moment and resumes about a second later. That is only when the program is ran by cron, not when I call it from the commandLine.</p> <p>Why is that so, is this perhaps due to the <code>usleep()</code> used in the program, and how can I make sure that the program execution isn't suspended for some time?</p> <p><strong>Update 1</strong>: Here is an extract with the usleep-section from the program</p> <pre><code>// Start reading while(NULL != (word = readToChar(fp, wordBuffer))) { // Values for the LEDs are stored in a struct updateValuesForLEDs(next, word); // Update struct "next" usleep(((next->time/timeFactor - lastTime)*1000000)); // Wait lastTime = next->time / timeFactor; // Set lastTime for next iteration setLEDs(i2cConnection, next, buffer); // Set the LEDs to the brightness values stored in struct "next" via I2C } </code></pre> <p><strong>Update 2:</strong> After ensc's comment I found way to solve my issue.</p> <p>The cronjob was started with a nice value of 10 whereas most other processes' values circled around 0. Setting my job to a lower nice value (which requires root privileges) gives it a higher priority and prevents it from being paused.</p> <pre><code>00 20 * * * sudo nice -n -20 ~/projects/startLEDShow >~/log.txt 2>&1 </code></pre>

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