[Openmcl-devel] time travel

Alexander Repenning ralex at cs.colorado.edu
Sat May 16 15:31:00 UTC 2009


thanks Gary,

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.

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.

(defun warp ()
   (declare (optimize (safety 2)))
   (dotimes (i 1000)
     (let ((t1 (#_mach_absolute_time)))
       (sleep 0.01)
       (let ((t2 (#_mach_absolute_time)))
         (when (> t1 t2) (print (- t2 t1)))))))

In CCL intel 64 the problem never seems to manifest itself.

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?

alex

On May 16, 2009, at 6:23 AM, Gary Byers wrote:

> AFAIK, mach_absolute_time() is implemented in terms of the  
> "rdtsc" (Read Time
> Stamp Counter") instruction, which returns the value of a 64-bit on- 
> chip
> counter that increments once every machine cycle.
>
> There are a few things that can make it difficult to use the value  
> returned
> by rdtsc for high-resolution timing.
>
> 1) the time-stamp counters are per-cpu, which generally means that  
> unless the
> OS takes steps to get and keep the TSCs of all CPU cores in sync,  
> the value
> returned by a rdtsc executed on CPU B may be less than a value  
> returned on
> CPU A.  As far as I know, OSX deals with this pretty well.
>
> 2) CPUs (especially those used in laptops and many consumer machines)
> don't always run at the same clock rate these days; they'll often
> switch into low-power states where the number of machine cycles per  
> second
> is less then the maximum.  On a Core2-Quad desktop machine running  
> Linux
>
> $ cat /sys/devices/system/cpu/cpu0/cpufreq/ 
> scaling_available_frequencies 2394000 1596000
>
> which says that a machine marketed as having a 2.4GHz CPU can also  
> run at
> a bit under 1.6GHz.
>
> Things like mach_absolute_time() have to be able to account for the  
> fact that
> the CPU frequency is dynamic (and therefore the values returned by  
> rdtsc have
> to be scaled by different amounts depending on the CPU frequency.)   
> I'm sure
> that mach_absolute_time() does deal with this, but I remember  
> looking at the
> code once and thinking that it was a lot more complicated than I  
> would have
> guessed.
>
> 3) [Even fuzzier.]  'rdtsc' instructions can be virtualized (meaning  
> that things
> like VMWare and Parallels can detect their use by at least some  
> programs.)  I
> don't know enough about the technology involved to know whether this  
> means that
> a 'hypervisor' (or whatever they call it) can affect the results  
> returned by
> rdtsc to host-OS programs.
>
> Both the result of the rdtsc instruction and the 64-bit integer
> returned by mach_absolute_time() are returned as a pair of 32-bit
> integers (containing the low and high 32 bits of the result.)  I find
> it awfully suspicious that the absolute value of difference between t2
> and t1 in the cases where time travel occurred is as close as it is to
> 2^32 (as if some code in mach_absolute_time() neglected to add a carry
> bit out of the low half into the high half.
>
> CCL just takes those 2 32-bit halves of mach_absolute_time()'s result
> and makes a lisp integer (almost always a bignum in the 32-bit lisp)
> out of them. (There -is- a bug here, in that 64-bit return values from
> foreign function calls aren't handled correctly in the x8632 CCL when
> the argument and return-value processing occur out of line; in your
> DOTIMES loop, that'd all happen in compiled code unless DEBUG or  
> SAFETY
> optimize settings are cranked up, and I don't think that there's a  
> problem
> when they aren't.
>
> So yes, I can think of a few explanations.  I have no idea if any of  
> them
> is correct, or what the workaround would be.
>
> FWIW, I can't get the DOTIMES loop below to fail unless I add a
> "(DECLARE (OPTIMIZE (SAFETY 3)))"; when it fails in that case, it's
> confused about the sign bit of the low 32 bits of the result (bit 31),
> and you're seeing apparent confusion about the value of the low bit of
> the high 32 bits (bit 32).
>
>
> On Fri, 15 May 2009, Alexander Repenning wrote:
>
>> In some animations I found irregularities which I traced back to  
>> some strange behavior of mach_absolute_time
>>
>> 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
>>
>> (dotimes (i 1000)
>> (let ((t1 (#_mach_absolute_time)))
>>  (sleep 0.01)
>>  (let ((t2 (#_mach_absolute_time)))
>>    (when (> t1 t2) (print (- t2 t1))))))
>>
>> -4284773073
>> -4284777830
>> -4284869530
>>
>> Can anybody reproduce this and is there any explanation, work  
>> around for this kind of madness?
>>
>> Alex
>>
>>
>> Prof. Alexander Repenning
>>
>> University of Colorado
>> Computer Science Department
>> Boulder, CO 80309-430
>>
>> vCard: http://www.cs.colorado.edu/~ralex/AlexanderRepenning.vcf
>>
>>
>

Prof. Alexander Repenning

University of Colorado
Computer Science Department
Boulder, CO 80309-430

vCard: http://www.cs.colorado.edu/~ralex/AlexanderRepenning.vcf


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