<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>thanks Gary,</div><div><br></div>I think you are right that the reported time warps are just suspiciously close to 2^32 nano seconds (~4.3 seconds) and the frequency of these time drift events is also matching, i.e., in the 10 second loop the problem appears 2-3 times on average.<div><br></div><div>I did add the less than max safety and it does appear to do the trick but just leaves me worried a bit that the "less save" code works better than the save one.<br><div><br></div><div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; ">(defun warp ()</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; "> (declare (optimize (safety 2)))</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; "> (dotimes (i 1000)</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; "> (let ((t1 (#_mach_absolute_time)))</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; "> (sleep 0.01)</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; "> (let ((t2 (#_mach_absolute_time)))</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; "> (when (> t1 t2) (print (- t2 t1)))))))</div><div><br></div><div>In CCL intel 64 the problem never seems to manifest itself.</div><div><br></div><div>For now I can get by with this solution. Is there any reason to think why this could change in the future again or could (declare (optimize (safety 2))) have any negative consequences in CCL 64?</div><div><br></div><div>alex</div><div><br><div><div>On May 16, 2009, at 6:23 AM, Gary Byers wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>AFAIK, mach_absolute_time() is implemented in terms of the "rdtsc" (Read Time<br>Stamp Counter") instruction, which returns the value of a 64-bit on-chip<br>counter that increments once every machine cycle.<br><br>There are a few things that can make it difficult to use the value returned<br>by rdtsc for high-resolution timing.<br><br>1) the time-stamp counters are per-cpu, which generally means that unless the<br>OS takes steps to get and keep the TSCs of all CPU cores in sync, the value<br>returned by a rdtsc executed on CPU B may be less than a value returned on<br>CPU A. As far as I know, OSX deals with this pretty well.<br><br>2) CPUs (especially those used in laptops and many consumer machines)<br>don't always run at the same clock rate these days; they'll often<br>switch into low-power states where the number of machine cycles per second<br>is less then the maximum. On a Core2-Quad desktop machine running Linux<br><br>$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies 2394000 1596000<br><br>which says that a machine marketed as having a 2.4GHz CPU can also run at<br>a bit under 1.6GHz.<br><br>Things like mach_absolute_time() have to be able to account for the fact that<br>the CPU frequency is dynamic (and therefore the values returned by rdtsc have<br>to be scaled by different amounts depending on the CPU frequency.) I'm sure<br>that mach_absolute_time() does deal with this, but I remember looking at the<br>code once and thinking that it was a lot more complicated than I would have<br>guessed.<br><br>3) [Even fuzzier.] 'rdtsc' instructions can be virtualized (meaning that things<br>like VMWare and Parallels can detect their use by at least some programs.) I<br>don't know enough about the technology involved to know whether this means that<br>a 'hypervisor' (or whatever they call it) can affect the results returned by<br>rdtsc to host-OS programs.<br><br>Both the result of the rdtsc instruction and the 64-bit integer<br>returned by mach_absolute_time() are returned as a pair of 32-bit<br>integers (containing the low and high 32 bits of the result.) I find<br>it awfully suspicious that the absolute value of difference between t2<br>and t1 in the cases where time travel occurred is as close as it is to<br>2^32 (as if some code in mach_absolute_time() neglected to add a carry<br>bit out of the low half into the high half.<br><br>CCL just takes those 2 32-bit halves of mach_absolute_time()'s result<br>and makes a lisp integer (almost always a bignum in the 32-bit lisp)<br>out of them. (There -is- a bug here, in that 64-bit return values from<br>foreign function calls aren't handled correctly in the x8632 CCL when<br>the argument and return-value processing occur out of line; in your<br>DOTIMES loop, that'd all happen in compiled code unless DEBUG or SAFETY<br>optimize settings are cranked up, and I don't think that there's a problem<br>when they aren't.<br><br>So yes, I can think of a few explanations. I have no idea if any of them<br>is correct, or what the workaround would be.<br><br>FWIW, I can't get the DOTIMES loop below to fail unless I add a<br>"(DECLARE (OPTIMIZE (SAFETY 3)))"; when it fails in that case, it's<br>confused about the sign bit of the low 32 bits of the result (bit 31),<br>and you're seeing apparent confusion about the value of the low bit of<br>the high 32 bits (bit 32).<br><br><br>On Fri, 15 May 2009, Alexander Repenning wrote:<br><br><blockquote type="cite">In some animations I found irregularities which I traced back to some strange behavior of mach_absolute_time<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">This loop should produce no output. On PPCs it works but on my Intel-Mac (with CCL 32) I get some output indicating that once in a while my computer appears to travel back in time for about 4 seconds<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">(dotimes (i 1000)<br></blockquote><blockquote type="cite">(let ((t1 (#_mach_absolute_time)))<br></blockquote><blockquote type="cite"> (sleep 0.01)<br></blockquote><blockquote type="cite"> (let ((t2 (#_mach_absolute_time)))<br></blockquote><blockquote type="cite"> (when (> t1 t2) (print (- t2 t1))))))<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">-4284773073<br></blockquote><blockquote type="cite">-4284777830<br></blockquote><blockquote type="cite">-4284869530<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Can anybody reproduce this and is there any explanation, work around for this kind of madness?<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Alex<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Prof. Alexander Repenning<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">University of Colorado<br></blockquote><blockquote type="cite">Computer Science Department<br></blockquote><blockquote type="cite">Boulder, CO 80309-430<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">vCard: <a href="http://www.cs.colorado.edu/~ralex/AlexanderRepenning.vcf">http://www.cs.colorado.edu/~ralex/AlexanderRepenning.vcf</a><br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><br></blockquote><br></div></blockquote></div><br><div apple-content-edited="true"> <div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><p style="margin: 0.0px 0.0px 0.0px 0.0px"><font face="Helvetica" size="3" style="font: 12.0px Helvetica">Prof. Alexander Repenning</font></p><p style="margin: 0.0px 0.0px 0.0px 0.0px"><br class="khtml-block-placeholder"></p><p style="margin: 0.0px 0.0px 0.0px 0.0px">University of Colorado</p><p style="margin: 0.0px 0.0px 0.0px 0.0px">Computer Science Department</p><p style="margin: 0.0px 0.0px 0.0px 0.0px">Boulder, CO 80309-430</p><p style="margin: 0.0px 0.0px 0.0px 0.0px"><br class="khtml-block-placeholder"></p><p style="margin: 0.0px 0.0px 0.0px 0.0px"><font face="Helvetica" size="3" style="font: 12.0px Helvetica">vCard: <a href="http://www.cs.colorado.edu/~ralex/AlexanderRepenning.vcf">http://www.cs.colorado.edu/~ralex/AlexanderRepenning.vcf</a></font></p><br class="Apple-interchange-newline"></span></div> </div><br></div></div></div></body></html>