[issue7753] newgil backport

[Antoine Pitrou]

Antoine Pitrou <pitrou at free.fr> added the comment:

> It appears to be better to use clock_gettime(CLOCK_MONOTONIC)
> where available and only use gettimeofday() as fallback solution
> together with times(), ftime() and time().

Ok, I've tried and it's less good than expected. Using CLOCK_MONOTONIC
absolutely kills efficiency. CLOCK_REALTIME is ok but it has no obvious
benefits (microsecond resolution as given by gettimeofday() is probably
sufficient).

The explanation AFAICT is that pthread_cond_timedwait() waits for
absolute clock values as given by CLOCK_REALTIME.
CLOCK_MONOTONIC gives other values (the man page says: "represents
monotonic time since some unspecified starting point"). These values are
probably "in the past" as seen from pthread_cond_timedwait(), which
implies a busy loop of waiting for the GIL to be released, inside of
being suspended gracefully until the timeout.

I can still produce a patch with only CLOCK_REALTIME but I'm not sure
it's worth the code complication.

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