On 12/04/2014 07:39 PM, Steven D'Aprano wrote:
Marko Rauhamaa wrote:
So, if I call
time.sleep(86400)
and the program is suspended for 24 hours, should time.sleep() return
right after it is resumed or after another 24 hours?
If the program is suspended, then no time should pass for that program.
Since sleep() is given in terms of a duration, not an absolute time ("sleep
until now + 24 hours"), if no time passes for that program, sleep() should
sleep for 24 hours *after the program resumes*.
Unfortunately a lot of systems get that wrong. E.g. I just ran "sleep 30"
from my Linux shell, immediately paused it using Ctrl-Z, waited a couple of
minutes, and used fg to continue. It returned immediately.
Why is this behaviour wrong?
Consider why people might call sleep. They're normally calling it to wait
for something else to complete. Often that something else is an external
process ("wait a few seconds to let the hard drive finish syncing, wait for
the web server to get less busy, wait for the user to catch up...") but it
might be another thread in the same process. If the process is suspended,
that other thread won't get to run, and so even though time on the outside
has continued to move forward, from the perspective of the process, it
hasn't.
The same applies even more so if the process is running in a virtual machine
which has just been suspended.
The same applies if the system clock is adjusted mid-sleep. If I call sleep
60, then immediately adjust the clock forward an hour (say, due to a
daylight savings adjustment), that shouldn't cause sleep to return. It
should still suspend for 60 seconds.
And I say you have it exactly backwards. People should NOT call sleep()
to measure something, they should only call it to release the processor
for "up to" a specified time. That time should normally be a major
fraction of the time that some external event is expected to take. Then
the code should check the external event and sleep some more if necessary.
If efficiency were not an issue, the code would just do a busy loop,
checking the external condition repeatedly.
Since the OS has no way of knowing whether the thing being waited for is
a thread, another process, a human being, a network operation, or the
end of the world, the interpretation of sleep needs to be the most
conservative one. There are many ways of suspending a process, and some
of them will also suspend the external event. Since the OS cannot know
which case is significant, it has to return control to the caller at the
soonest of the many possible interpretations.
--
DaveA
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