Premature wakeup of time.sleep()

[Erich Schreiber]

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?

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? 

Regards,

Erich Schreiber