Re: Premature wakeup of time.sleep()

Erich Schreiber wrote: 
In the Python Library Reference the explanation of the time.sleep()
function reads amongst others:


The actual suspension time may be less than that requested because any caught signal will terminate the sleep() following execution of that signal's catching routine. Also, the suspension time may be longer than requested by an arbitrary amount because of the scheduling of other activity in the system. 


I don't understand the first part of this passage with the premature
wakeup. What signals would that be?

That would be signals from the OS, frinstance, if you type ^C into the python console, or kill the process from another command line.


I've written a script that tries to bench the responsiveness of a
virtual Linux server. My script sleeps for a random time and on waking
up calculates the difference of the proposed and actual wakeup time.

The essential code fragment is

    while True:
        ts = tDelay()
        t1 = time.time()
        time.sleep(ts)
        t2 = time.time()
        twu = str(datetime.datetime.utcfromtimestamp(t1 + ts))
        logResults(LOGFILE, twu, ts, int((t2-t1-ts)*1000))

Whereas tDelay() returns a (logarithmically) randomly distributed real
number in the range [0.01, 1200] which causes the process to sleep
from 10 ms to 20 minutes.

In the logs I see a about 1% of the wake-up delays beeing negative
from -1ms to about -20ms somewhat correlated with the duration of the
sleep. 20 minute sleeps tend to wake-up earlier then sub-second
sleeps. Can somebody explain this to me?


I think the sleep times are quantised to the granularity of the system clock, shich varies from os to os. From memory, windows 95 has a 55mS timer, NT is less (19mS?), Linux and solaris 1mS. All this is from memory, and really comes from discussions I have seen about sleep and time in java, but I am guessing that there are similarities.

Steve
.

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