Q: What is Timer actually observing

[mwpowellhtx]

Would someone mind clarifying, Timer, or any of the timer based extensions. What is long exactly?

/// ...
/// <returns>An observable sequence that produces a value after the due time has elapsed.</returns>
///                                 ^^^^^^^^^^^^^^^^^^^^^
public static IObservable<long> Timer(TimeSpan dueTime) { ... }

Is that just a random long value? Number of ticks? What exactly...

[mwpowellhtx]

So from inferred analysis, appears as though long x is reporting the number of steps or iterations the timer has performed.

So... when that timer has achieved 9,223,372,036,854,775,807, what happens next? Does that roll over to zero (0)?

Or do we need to think about canceling the previous subscription and restarting the timer, if it was intended to run long, pardoning the pun.

[idg10]

It is, as you observed, just a sequence number: it goes up by one each time.

when that timer has achieved 9,223,372,036,854,775,807

This would require over 9 quintillion iterations. A Timer generating 1 billion events per second would need to run for about 292 years to generate that many ticks.

In practice Timer can't go that fast on current hardware. And even though Moore's Law continues to press on, we no longer see exponential improvements in single-threaded execution speed. (That stopped about 20 years ago, although apparently most futurologists have yet to notice this.) Since Rx's IObserver<T> contract is inherently single threaded (the Timer is not allowed to make a new call to OnNext before the previous one returns) this single-threaded performance is the limiting factor.

I believe Rx's fastest schedule for this particular scenario is the EventLoopScheduler. On my machine, the fastest a Timer can run seems to be about 6 million events per second, which means the Timer would run for over 48,000 years before overflowing.

Given the normal service life of a computer, it seems vanishingly unlikely that any program started today will still be running 48 thousand years from now without a reboot. It tends to be a major project to get computers from just 50 years ago to run. And even if you think today's hardware is capable of running for that long, history suggests that today's civilisations will all fail long before the Timer count overflows.

Planning for this eventuality therefore seems like overengineering. So no, I don't think it's necessary in practice to think about cancelling and restarting the subscription.

[mwpowellhtx]

Right eventually I did a little first if second order math and figured, probably never going to reach that number for practical purposes... Still, nice to know and would be easy I think to perhaps test the edge cases, initialize to MAX-1 for instance.