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POtime.clock and time.strftime Give Totally Different Results
 

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  1. POtime.clock and time.strftime Give Totally Different Results
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    <p>My program takes a measurement once per loop and sleeps for a specified amount, in this case 10 seconds. It also measures time. It measures the time in two ways. Once with time.strftime, and another with time.clock(). On most of my computers, these results are totally consistent and I have no problems. On one computer, they don't agree at all. </p> <p>Here's the code segment in question. In my program it was running independently in three threads. self.delay is a float holding the user-specified delay, in this case 10.0 seconds.</p> <pre><code> cycles = 0 startTime = time.clock() while(blah) cycleBeginTime = time.clock() ... t = time.strftime("%Y-%m-%d %H:%M:%S") ... cycles += 1 cycleEndTime = time.clock() wakeUp = startTime + cycles * self.delay if cycleEndTime &gt; wakeUp: #we overslept continue else: #not guaranteed to sleep for the exact specified amount of time time.sleep(float(wakeUp - cycleEndTime)) afterSleepTime = time.clock() print ("sleep(" + str(wakeUp - cycleEndTime) + ") lasted " + str(afterSleepTime - cycleEndTime) +" seconds\n" + "Total time for this cycle: " + str(afterSleepTime - cycleBeginTime) + "\ntime from start of cycle to sleep " + str(cycleEndTime-cycleBeginTime) ) </code></pre> <p>Here's the results on the console for the time as measured with time.clock. Skip ahead to the next part for a summary.</p> <pre><code>sleep(9.8107975515) lasted 4.31354512806 seconds Total time for this cycle: 4.50274184753 time from start of cycle to sleep 0.189196719463 sleep(9.83803382537) lasted 4.35964227608 seconds Total time for this cycle: 4.5216022856 time from start of cycle to sleep 0.161960009523 sleep(9.83973893539) lasted 4.36409510551 seconds Total time for this cycle: 4.52435043356 time from start of cycle to sleep 0.160255328054 sleep(15.3537603228) lasted 5.42625884166 seconds Total time for this cycle: 5.56417636459 time from start of cycle to sleep 0.137917522931 sleep(15.3879203849) lasted 5.45131225502 seconds Total time for this cycle: 5.5384287755 time from start of cycle to sleep 0.0871165204752 sleep(15.3801304296) lasted 5.45686671345 seconds Total time for this cycle: 5.55443364994 time from start of cycle to sleep 0.0975669364944 sleep(19.7024141096) lasted 2.5903386547 seconds Total time for this cycle: 2.81485116786 time from start of cycle to sleep 0.224512513157 sleep(19.7236584582) lasted 2.61606277881 seconds Total time for this cycle: 2.81505236078 time from start of cycle to sleep 0.198989581976 sleep(19.7569903213) lasted 2.64424758408 seconds Total time for this cycle: 2.8228942459 time from start of cycle to sleep 0.178646661821 sleep(26.8608515814) lasted 3.1923968974 seconds Total time for this cycle: 3.44044448649 time from start of cycle to sleep 0.248047589093 sleep(26.9264651571) lasted 3.24803654453 seconds Total time for this cycle: 3.42464766929 time from start of cycle to sleep 0.176611124756 sleep(26.9123819307) lasted 6.19344847627 seconds Total time for this cycle: 6.39064386998 time from start of cycle to sleep 0.197195393715 sleep(30.50445713) lasted 11.3544706882 seconds Total time for this cycle: 11.5452852063 time from start of cycle to sleep 0.190814518069 sleep(30.5479180492) lasted 11.4011029222 seconds Total time for this cycle: 11.5583578442 time from start of cycle to sleep 0.157254922059 sleep(30.5384771841) lasted 11.3943939803 seconds Total time for this cycle: 11.5739287254 time from start of cycle to sleep 0.179534745126 sleep(29.032023896) lasted 9.57638019147 seconds Total time for this cycle: 9.6907935091 time from start of cycle to sleep 0.114413317628 sleep(28.9997437096) lasted 9.55454254173 seconds Total time for this cycle: 9.70431450053 time from start of cycle to sleep 0.149771958799 sleep(29.0315669415) lasted 9.57838381284 seconds Total time for this cycle: 9.69044695504 time from start of cycle to sleep 0.112063142198 sleep(29.2684610421) lasted 11.5343751591 seconds Total time for this cycle: 11.7100907949 time from start of cycle to sleep 0.175715635808 sleep(29.4380200767) lasted 11.7063676658 seconds Total time for this cycle: 11.7231073229 time from start of cycle to sleep 0.01673965716 sleep(29.2840066914) lasted 11.5395576362 seconds Total time for this cycle: 11.7081641622 time from start of cycle to sleep 0.168606525989 </code></pre> <p>Here's a summary of the timestamps taken with time.strftime, as compared with the measurements from time.clock and the attempted sleep time.</p> <pre><code>2012-04-04 17:22:07 2012-04-04 17:22:17 (diff 10s. Attempted sleep time 10s time.clock says 4.5s) 2012-04-04 17:22:32 (diff 15s. Attempted sleep time 15.4s time.clock says 5.4s) 2012-04-04 17:22:52 (diff 20s. Attempted sleep time 19.7s time.clock says 2.8s) 2012-04-04 17:23:46 (diff 54s. Attempted sleep time 27s time.clock says 3.4s) 2012-04-04 17:24:16 (diff 30s. Attempted sleep time 30.5s time.clock says 11.5s) 2012-04-04 17:24:45 (diff 29s. Attempted sleep time 29s time.clock says 9.7s) 2012-04-04 17:25:15 (diff 30s. Attempted sleep time 29.4s time.clock says 11.7s) </code></pre> <p>As you can see, time.strftime agrees with sleep most, but not all of the time (they disagree on 2012-04-04 17:23:46), while time.clock gives completely bogus nonsense all the time. Is there something that would cause these two functions to give complete nonsense results?</p> <p>EDIT: totally rewote post for brevity.</p> <p>EDIT: Well, I have a solution, but I don't understand why. <a href="http://docs.python.org/library/time.html" rel="nofollow">This page</a> says that time.clock() gives wall-clock time when used under Windows, but CPU-time when used under Unix. One of the answerers who has since deleted his post, mistakenly said that time.clock() is CPU-time, but I wonder if he may be correct in spite of the docs. When I replaced all my calls to time.clock() with calls to time.time(), my program now works even on the problem computer.</p>
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    1. COWhat does the output look like on a computer where it works?
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    2. COOn a computer where it works, time.clock's measurement is within about 10 milliseconds of the specified sleep time, and sleep, time.clock(), and time.strftime all agree with one another.
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    3. CODid you install stock CPython right from http://python.org or is it some distribution that might be doing something different with `time.clock`? What version of Python?
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