[tex-live] strange discrepancy in running time of etex between TL2015 and TL2017

[Reinhard Kotucha]

On 2017-07-28 at 09:15:32 +0200, jfbu wrote:

 > I have lots of experimenting with \pdfelapsedtime on Mac OS and
 > also a bit on Linux, and it has always stricken me as quite
 > fluctuating even when used for durations of tens of seconds.

 > One such source is definitely deep in the CPU management, because
 > on my laptop I have a specific phenomenon which I do not observe on
 > a Mac desktop regarding computation times when running into
 > minutes: for very lengthy things (5mns+) my laptop *slows down* in
 > comparison to the desktop or the Linux machine

Hi Jean-Francois,
the CPU fan is obviously not able to keep the CPU temperature below a
certain limit.  In order to prevent the CPU from being destroyed by
heat, the clock frequency is reduced.  This is intended and expected
behavior.

Power consumption increases linearly with clock frequency.  If your
CPU gets too hot, your system has only two choices: either reduce the
clock frequency or shutdown the system completely.

You can check yourself:  Before starting your program, open another
terminal window and run

  while true; do clear; grep 'cpu MHz' /proc/cpuinfo; sleep 1; done

therein.  This works at least on Linux.  On OS X you probably have to
run "sysctl -a" and check whether the current CPU frequencies are
reported.  I don't have OS X.

I suppose that the CPU frequencies are quite low at the beginnig, when
no other processes are running.  When you start your program, at least
one CPU will run at full speed.  After a while, when your system gets
slower, you'll see that CPU frequencies are decreased.  You'll also
notice that the air coming from the CPU fan is extremely hot.

The problem is that laptops are almost as powerful as desktop PCs but
there is not enough space for a decent cooling system.  This is quite
unfortunate because lifetime of electronic components heavily depend
on temperature.

BTW, I encountered a peculiarity with my Linux machine a few years
ago.  I could do everything with this machine -- except compiling gcc.
Whenever I invoked "./configure && make -j3", the power supply turned
off after a few minutes.  In this case the power supply was too weak
and replacing it by a bigger one solved the problem.

Regards,
  Reinhard

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Reinhard Kotucha                            Phone: +49-511-3373112
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