Why does node/io.js hang when printing to the console inside a loop?

[pgkos]

Sorry if it is a dumb question/issue, but why does this code:

for (i = 0; i < 1000000; i++) {
  console.log('abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc' + i);
}

hang node for a few seconds (at around i = 1500) before resuming again? During the hang the amount of memory used by the node process increases very fast and the CPU usage is very high during that time.

On the other hand, if I write:

for (i = 0; i < 1000000; i++) {
  console.log(i);
}

then the node process does not hang at all.

The reason I am asking is that I have a piece of code which involves printing inside a loop, and it also hangs, but for much longer (like 30-60 seconds, before resuming again). The code prints the paths of all files in a directory (walking recursively): https://gist.github.com/pgkos/f0a650daf56aa49899e9.

The issue occurs on both io.js v2.0.2 and node.js v0.12.2, on Ubuntu 64-bit.

[bnoordhuis]

I can think of a couple of reasons. The first snippet creates a million strings (i.e. does a million heap allocations) whereas the second one does not; you can verify that with the --trace_gc flag.

What's more, if the first argument to console.log() is a string, it's considered a format string that needs to be parsed. In the first snippet, that works out to about 60 MB of memory that is scanned. You can avoid that by using process.stdout.write() instead.

Apropos memory usage, console.log() and process.stdout.write() are mostly asynchronous under the hood. When you call it, a 'write to the console' request is put together that is then handed off to libuv.

It's not until the next tick of the event loop that the memory of that request is freed; that includes the actual string that is written. In both snippets, one million requests are in flight but in the first one, they're much bigger.

[ChALkeR]

Actually, + i is not needed.there.

var x = process.hrtime();
for (var i = 0; i < 1000000; i++) {
    console.log('abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc1234567890');
}
console.log(process.hrtime(x));

Is almost equally slow.

[ChALkeR]

@bnoordhuis

What's more, if the first argument to console.log() is a string, it's considered a format string that needs to be parsed. In the first snippet, that works out to about 60 MB of memory that is scanned. You can avoid that by using process.stdout.write() instead.

See #1749. It does not fix this issue, but makes things slightly better.

[Fishrock123]

Simple answer: the code in question blocks the process for a bit while the loop is running~~, and libuv doesn't print it until the next tick~~.

[ChALkeR]

@Fishrock123 I would say that (almost) everything is printed in the same tick.

The cause of the lag is gc going crazy at some point and hanging everything for a second or so.

Part of this single gc loop:

[16372]      251 ms: Scavenge 9.3 (45.0) -> 6.8 (46.0) MB, 2.3 ms [allocation failure].
[16372]      263 ms: Scavenge 9.8 (46.0) -> 7.7 (47.0) MB, 3.2 ms [allocation failure].
[16372]      403 ms: Scavenge 10.8 (47.0) -> 8.6 (47.0) MB, 132.1 ms [allocation failure].
[16372]      428 ms: Scavenge 15.6 (47.0) -> 10.6 (48.0) MB, 5.6 ms [allocation failure].
[16372]      452 ms: Scavenge 16.5 (48.0) -> 12.3 (50.0) MB, 7.6 ms [allocation failure].
[16372]      738 ms: Scavenge 18.5 (50.0) -> 14.1 (52.0) MB, 254.8 ms (+ 13.7 ms in 94 steps since last GC) [allocation failure].
[16372]      795 ms: Mark-sweep 22.0 (52.0) -> 15.7 (54.0) MB, 15.8 ms (+ 32.0 ms in 214 steps since start of marking, biggest step 0.4 ms) [GC interrupt] [GC in old space requested].
[16372]      846 ms: Scavenge 29.2 (54.0) -> 19.7 (56.0) MB, 11.0 ms [allocation failure].
[16372]      892 ms: Scavenge 31.5 (56.0) -> 23.1 (60.0) MB, 12.9 ms [allocation failure].
[16372]      934 ms: Scavenge 35.4 (60.0) -> 26.7 (63.0) MB, 12.4 ms [allocation failure].
[16372]      976 ms: Scavenge 38.8 (63.0) -> 30.2 (67.0) MB, 13.1 ms [allocation failure].
[16372]     1019 ms: Scavenge 42.4 (67.0) -> 33.7 (71.0) MB, 12.9 ms [allocation failure].
[16372]     1060 ms: Scavenge 45.9 (71.0) -> 37.3 (74.0) MB, 12.3 ms [allocation failure].
[16372]     1103 ms: Scavenge 49.5 (74.0) -> 40.8 (78.0) MB, 12.5 ms [allocation failure].
[16372]     1145 ms: Scavenge 53.0 (78.0) -> 44.3 (81.0) MB, 13.1 ms [allocation failure].
[16372]     1187 ms: Scavenge 56.5 (81.0) -> 47.9 (85.0) MB, 12.5 ms [allocation failure].
[16372]     1229 ms: Scavenge 60.1 (85.0) -> 51.4 (89.0) MB, 12.7 ms [allocation failure].
[16372]     1272 ms: Scavenge 63.6 (89.0) -> 54.9 (92.0) MB, 12.7 ms [allocation failure].
[16372]     1315 ms: Scavenge 67.1 (92.0) -> 58.5 (96.0) MB, 12.7 ms [allocation failure].
[16372]     1388 ms: Scavenge 70.7 (96.0) -> 62.0 (99.0) MB, 15.9 ms (+ 24.6 ms in 186 steps since last GC) [allocation failure].
[16372]     1533 ms: Scavenge 74.2 (99.0) -> 65.6 (103.0) MB, 19.9 ms (+ 91.8 ms in 186 steps since last GC) [allocation failure].
[16372]     1611 ms: Scavenge 77.8 (103.0) -> 69.1 (107.0) MB, 16.2 ms (+ 28.4 ms in 186 steps since last GC) [allocation failure].
[16372]     1683 ms: Scavenge 81.3 (107.0) -> 72.6 (110.0) MB, 13.5 ms (+ 26.9 ms in 186 steps since last GC) [allocation failure].
[16372]     1755 ms: Scavenge 84.8 (110.0) -> 76.2 (114.0) MB, 13.7 ms (+ 25.6 ms in 186 steps since last GC) [allocation failure].
[16372]     1828 ms: Mark-sweep 80.9 (114.0) -> 77.5 (117.0) MB, 49.2 ms (+ 208.7 ms in 1003 steps since start of marking, biggest step 68.1 ms) [GC interrupt] [GC in old space requested].
[16372]     1874 ms: Scavenge 91.8 (117.0) -> 81.6 (119.0) MB, 10.7 ms [allocation failure].
[16372]     1917 ms: Scavenge 93.2 (119.0) -> 85.0 (123.0) MB, 15.0 ms [allocation failure].

[ChALkeR]

@Fishrock123 FYI, I can reproduce this in async:

function f(max) {
    for (var i = 0; i < max; i++) {
        console.log('abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc1234567890');
    }
}

function g() {
    f(100);
    process.nextTick(g);
}
g();

Run with --trace-gc.

[ChALkeR]

Btw, I saw misconceptions about sync code.
Please note:

1. It does not block any type of gc runs from executing. Both scavenge and mark-sweep runs are automatically executed in the middle of a tick. No need to manually trigger gc().
2. It does not block output by itself.

[ChALkeR]

Another example:

function g() {
    process.stdout.write('abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc1234567890\n');
    process.nextTick(g);
}
g();

It's even far worse in this case. When gc starts, it never finishes running (runs over and over again with no actual stdout output), memory usage quickly goes to 1.4 GiB and the process quits with «Allocation failed - process out of memory» error.

It looks as if garbage collector blocks output from happening, but the js code still runs, process.stdout.write gets called and overflows the output buffer.

[ChALkeR]

If you don't like recursive process.nextTick, let's setTimeout for 1ms every 1e6 iterations.

var i = 0;
function g() {
    console.log('abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc1234567890');
    i++;
    if (i % 1000000 === 0) {
        setTimeout(g, 1);
    } else {
        process.nextTick(g);
    }
}
g();

Takes more time, but still crashes.

Another example (setTimeout for 0 ms every 1e5 iterations):

var i = 0;
function g() {
    console.log('abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc1234567890');
    i++;
    if (i % 100000 === 0) {
        setTimeout(g, 0);
    } else {
        process.nextTick(g);
    }
}
g();

Another one:

var s = 'abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc1234567890';
function g() {
    for (var i = 0; i < 1e5; i++) {
        console.log(s);
    }
    setTimeout(g, 0);
}
g();

The numbers might require some tuning if you have a system that is faster than mine.

[ChALkeR]

Asked on IRC by @Fishrock123 :

can you try allocating the sting outside the loop and using a reference to it?

Doesn't change anything