timedrift.js

timedrift.js

"use strict";

function millis(hr) {
	return hr[0] * 1000 + Math.floor(hr[1] / 1000000);
}

function busyWait(ms, cb) {
	var hr0 = process.hrtime();
	while (millis(process.hrtime(hr0)) < ms);
	var elapsed = millis(process.hrtime(hr0));
	console.log("busy:\texpected " + ms + ", got " + elapsed);
	cb();
}

function idleWait(ms, cb) {
	var hr0 = process.hrtime();
	setTimeout(function() {
	var elapsed = millis(process.hrtime(hr0));
		console.log("idle:\texpected " + ms + ", got " + elapsed + //
			(elapsed > ms + 100 ? " SURPRISE!!" : ""));
		cb();
	}, ms);
}

function combined(a) {
    busyWait(300, function() {
        idleWait(100,function(){
            a < 50 && combined(++a);
        });
    }); 
}
combined(0);

Output:

busy:   expected 300, got 300
idle:   expected 100, got 102
busy:   expected 300, got 300
idle:   expected 100, got 402 SURPRISE!!
busy:   expected 300, got 300
idle:   expected 100, got 401 SURPRISE!!
busy:   expected 300, got 300
idle:   expected 100, got 401 SURPRISE!!
DONE: 2516

I update to measure times with process.hrtime rather than Date.now. Same results

the fact that you get a drift exactly equal to the amount of time you sleep/idle looks suspiciously like the error i get using chained setTimeout()'s. if you want to read my gist, you'll see somehow using clearTimeout() eradicates the drift. https://gist.github.com/natbro/bf872ac473a1bba04008