Iterators 'blocking' each other

[jeswr]

I suspect that this may be caused by #35 so don't worry for now.

---

This is a potential issue that I saw when perf testing the current PRs, but also saw in main.

If I run (on the main branch)

import { ArrayIterator, range } from '../dist/asynciterator.js'

for (let i = 0; i < 5; i++) {
  const arr = new range(0, 2000000); // I've tested with both `range` and `ArrayIterator`
  
  const iterator = arr
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item);
  
  const now = performance.now();
  iterator.on('data', () => {}).on('end', () => {
    const end = performance.now();
    console.log('elapsed', end - now);
  });
}

then the result is

elapsed 14108.122984007001
elapsed 14112.1332180053
elapsed 14112.153782993555
elapsed 14112.245641991496
elapsed 14112.30153799057

(as opposed to ~3000ms for one)

I suspect the issue is that the end event is called asynchronously so has to wait for everything else to complete before we get back to it.

[jacoscaz]

I'm not entirely sure about my assumptions here but this should generate a significant amount of CPU contention between iterators as we're still single-threaded (-ish, I know there's some magic happening in V8) and there's no I/O anywhere.

On my machine, the time per item remains constant and the total time scales linearly with the overall number of items, regardless of the number of (quasi-)parallel chains. I get between 8.8 and 9.3 seconds for 10_000_000 items regardless of whether that's due to 1 chain going through 10_000_000 items or 5 chains going through 2_000_000 items each.

Curiously, distributing 10_000_000 items across 100 chains going through 100_000 items each results in roughly 1.25x faster performance, finishing in ~ 6 seconds. I don't think it's something to do with the end event.

[jeswr]

Just ran some tests that seem to support what @jacoscaz is saying here

Experiment

import { ArrayIterator, range, AsyncIterator } from 'asynciterator'

for (let i = 0; i < 5; i++) {
  const arr = range(0, 2000000); // I've tested with both `range` and `ArrayIterator`
  
  const iterator = arr
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item)
    .map((item) => item);
  
  console.time(`End event emitted for iterator ${i}`)
  console.time(`First data event emitted for iterator ${i}`)
  console.time(`Middle data event emitted for iterator ${i}`)
  console.time(`Last data event emitted for iterator ${i}`)
  iterator.on('data', (x) => {
    if (x === 0) {
      console.timeEnd(`First data event emitted for iterator ${i}`)
    }
    if (x === 1000000) {
      console.timeEnd(`Middle data event emitted for iterator ${i}`)
    }
    if (x === 2000000 - 1) {
      console.timeEnd(`Last data event emitted for iterator ${i}`)
    }
  }).on('end', () => {
    console.timeEnd(`End event emitted for iterator ${i}`)
  });
}

Results

First data event emitted for iterator 0: 8.585ms
First data event emitted for iterator 1: 9.589ms
First data event emitted for iterator 2: 9.729ms
First data event emitted for iterator 3: 9.942ms
First data event emitted for iterator 4: 10.167ms
Middle data event emitted for iterator 0: 7.531s
Middle data event emitted for iterator 1: 7.531s
Middle data event emitted for iterator 2: 7.530s
Middle data event emitted for iterator 3: 7.531s
Middle data event emitted for iterator 4: 7.531s
Last data event emitted for iterator 0: 14.690s
Last data event emitted for iterator 1: 14.690s
Last data event emitted for iterator 2: 14.690s
Last data event emitted for iterator 3: 14.690s
Last data event emitted for iterator 4: 14.690s
End event emitted for iterator 0: 14.691s
End event emitted for iterator 1: 14.691s
End event emitted for iterator 2: 14.691s
End event emitted for iterator 3: 14.691s
End event emitted for iterator 4: 14.691s