Re: after ids

Donal K. Fellows wrote:
OK, how do you work out how long until the next event fires without an
absolute time source? For added bonus points, do so with arbitrary
real-time delays between processing points and without extra threads.

Donal,

Many people here are unquestionably arguing from ignorance, but
I do suspect after some conversations with Alexandre Ferrieux
that they are onto something with wanting a distinction between
absolute and relative times.

Absolute time, as far as Tcl is concerned, is UTC with smoothing
(http://www.cl.cam.ac.uk/~mgk25/uts.txt). It advances monotonically
and (more or less) continuously, ticking at an uneven rate to
accommodate TAI-UTC differences and NTP corrections. It *can*
be nonmonotonic or discontinuous in unusual circumstances, owing
to an act of God (a network outage causing too large an NTP jump
to recover by advancing or retarding clock frequency), an act
of the operator (adjusting system time by the wristwatch-and-
-eyeball method), or an Act of Congress (Daylight Saving Time,
on operating systems that use civil time as the reference).

Its reference on Unix-like systems is gettimeofday(); on Windows
systems its ultimate reference is the system clock returned
by GetSystemTimeAsFileTime, with additional precision provided
by interpolation using the performace counter.

Relative time is not something that Tcl currently recognises,
but perhaps should. It is strictly monotonic and uniform;
it can be thought of as the independent variable of Newton's
Laws of Motion. It is not - indeed, cannot be - tied to any
concept of absolute time outside the computer, since all the
"acts" above can cause dislocations. Nevertheless, it appears
to be what people want in scheduling short-period tasks.
Moreover, the anomalies in time handling on Windows (where
near the Spring and Autumn transitions of Daylight Saving Time,
the "UTC" returned by the system can be an hour off!) appear
to be with us to stay. I've seen bugs in that area in every
Windows release from 3.1 to Vista.

We have historically (and I am an offender in that respect)
offered the answer, "make your NTP infrastructure work!"
That's certainly part of the answer, and necessary for other
reasons (such as making sure that the system clock is
reasonably synchronised with the clock of remotely-mounted
file systems). It does not address the fact that there
are timing windows at the Daylight Saving Time transitions
during which the Windows API returns times that are simply
incorrect. It also fails to address the concerns of those
who deal with systems that have only intermittent network
connectivity or lack it altogether. (Tcl gets ported to
some very strange places, indeed.)

Given these considerations, I think we can all agree that,
all else being equal, we might gain something by
distinguishing the relative timing function (today's
[after] used for short-period tasks with loose accuracy
being tolerable) and the absolute timing function (delivery
of an event at a given time in the future, accounting
for any known dislocations of the clock). The latter might
be implemented by a command analogous to 'after' that
accepts seconds (milliseconds, microseconds, choose
a convenient unit) of UTS time from the epoch instead
of milliseconds from the current time.

The chief argument against that approach, we have stated
in the past, is infeasibility. Unquestionably, we have
system calls that delay a certain length of time irrespective
of changes to the civil clock - indeed, sleep() and
select() on Unix; and Sleep and MsgWaitForMultipleObjectsEx
on Windows work that way. What we have lacked, or so we
have claimed, is a reliable way to handle multiple
relative timers - once the first timer has rung, how
do we wind the second timer with the now-shorter interval
that it needs? What is needed is a reference clock that
advances with the properties that we require (monotonicity,
uniformity, but not accuracy nor synchrony with the
outside world).

I - and apparently you, Donal - had long believed that
no portable way existed to get such a clock, but I am
coming to suspect that the world is catching up with our
needs. On Windows, the information is available with
the GetTickCount function, which has existed since the
beginning (Windows attempts to calibrate its rate to
NTP corrections, but guarantees its monotonicity
except for the 49.7-day rollover, which we could deal
with easily). Unix-like systems are a tougher nut
to crack, but with the Posix standards being implemented
more and more widely, I suspect that the times() function
is a reasonable reference on Unix-like systems. While
its primary purpose is to retrieve CPU time for a process,
it has the side effect of returning clock ticks since
an arbitrary point in the past, advancing in monotonic
fashion. (This reference is needed when successive
calls to times() are used to compute percent-CPU-usage.)

It remains to be seen whether the combination of
times() and GetTickCount() will work on all the platforms
that we support, but it's also something that we could
exploit anyway, by having the configurator check for
the routines and fall back upon today's usage of
absolute time if they are not available.

I don't personally have the time at the moment to
tackle such a project, but if someone else wants to
draft a TIP and attempt a reference implementation,
I'd be willing to shepherd them through the process.

--
73 de ke9tv/2, Kevin
.

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