Sagar V Sagar V -4 years ago 67
Javascript Question

Why the huge time difference between while and do..while in JavaScript

The while loop



Test the condition and if it is true, then execute the code

The do..while loop



Execute first time. Then test and execute.

So the difference between
while
and
do..while
is , programmatically In while, one test is carried out more than do while

That is

If a loop from 1 to 50 execute in
while
loop with one statement, it will have 51 tests(50 true and 1 false) and the statement will execute 50 times.

Similarly

If a loop from 1 to 50 execute in
do..while
loop with one statement, it will have 50 tests(1st test will not be carried out) and the statement will execute 50 times.

So, only one test/check less. that's it.

But when I tested the time taken for executing, it shows large difference.



function whileFn() {
var i = 0;
while (i < 10) {
document.write(i);
i++;
}
}

function doWhileFn() {
var i = 0;
do {
document.write(i);
i++;
} while (i < 10)
}

console.time('whileFn');
whileFn();
console.timeEnd('whileFn');

document.write('<br/>');

console.time('doWhileFn');
doWhileFn();
console.timeEnd('doWhileFn');





As you can see on the image and code exampe, the
while
loop took 15ms where as
do while
took only 5 ms.

What is the reason behind this huge different?

Test for 10 elements

enter image description here

Update as suggested by @pid



Test for 1000

enter image description here

took 23mS for 1 extra test

Test for 10000

enter image description here

397.91 mS more for 1 extra test

Test is carried on


Chrome (58.0.3029.110)

Answer Source

EDIT: I HAVE AN ANSWER (TL;DR: SKIP TO THE END)

I've done some tests on my own.

function whileFn() {
  var i = 0;
  while (i < 10) {
    document.write(i);
    i++;
  }
}

function doWhileFn() {
  var i = 0;
  do {
    document.write(i);
    i++;
  } while (i < 10)
}


console.time('doWhileFn');
doWhileFn();
console.timeEnd('doWhileFn');

document.write('<br/>');

console.time('whileFn');
whileFn();
console.timeEnd('whileFn');

I've inverted the two functions and the timing is still the same. That is, the first is always slower than the second one. This is proof that the loop has no meaning whatsoever, it is completely bound by the rendering engine. (rendering is irrelevant)

If you remove document.write() altogether, the difference is reduced even more. (irrelevant)

To correctly measure the time, you have to take into account the measurement of time itself, in fact this shows the overhead of measuring time:

console.time('outer');
console.time('inner');
for (var i = 0; i < 10; i++);
console.timeEnd('inner');
console.timeEnd('outer');

The difference between the inner and outer measurement is a measurement overhead and impacts on the measurement itself (Heisenberg anyone?) so much that timing very fast functions (next to the ms mark) is prone to measurement errors. TRUE BUT IRRELEVANT

Try wrapping your code in huge cycles (like repeat 1000-100000 times) to reduce the impact of measurement. THIS PROVES TO BE NOT THE CASE

By the above statement long cycles would have a tiny measurement difference, but tests show that the difference scales with the number of cycles, and as such is NOT just a measurement overhead.

To recap the findings so far:

  • it is not a matter of while and do..while, because inverting the order of the two functions does not invert the timing: the first always is the slower one;
  • it is not a matter of measurement overhead because the difference scales to macroscopic proportions (it should be a variable, yet tiny amount -- but it's not);
  • it is not about rendering because I've removed it altogether at some point;
  • the inner-outer snippet shows that long cycles have a tiny measurement overhead by replacing 10 with a large number, but this is not the case for the original code in the question -- here the difference is proportional to the number of cycles.

EDIT: conclusion

This is an alternating test. Measure A, B, A again, B again and finally A again: the more you move forward, the more it converges.

Proof:

function whileFn() {
  var i = 0;
  while (i < 10) {
    document.write(i);
    i++;
  }
}

function doWhileFn() {
  var i = 0;
  do {
    document.write(i);
    i++;
  } while (i < 10)
}


console.time('doWhileFn');
doWhileFn();
console.timeEnd('doWhileFn');

document.write('<br/>');

console.time('whileFn');
whileFn();
console.timeEnd('whileFn');

document.write('<br/>');

console.time('doWhileFn');
doWhileFn();
console.timeEnd('doWhileFn');

document.write('<br/>');

console.time('whileFn');
whileFn();
console.timeEnd('whileFn');

document.write('<br/>');

console.time('doWhileFn');
doWhileFn();
console.timeEnd('doWhileFn');

Explanation: the JS engine compiles the source JS into native code on-the-fly. It has gradual performance scaling, but it can only compile a function AFTER it has returned. This means that the function is compiled and gradually optimized over a longer period of time. This, in fact, is a well known feature of V8. What is measured in the A-B scenario is not representative because of this edge condition (initial measures are inaccurate). The A-B-A-B-A scenario shows that A and B converge over time and measurements settle when they are far away from the edge (initial) condition.

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