stdout/stderr buffering considerations

[ChALkeR]

I tried to discuss this some time ago at IRC, but postponed it for quite a long time. Also I started the discussion of this in #1741, but I would like to extract the more specific discussion to a separate issue.

I could miss some details, but will try to give a quick overview here.

Several issues here:

1. Many calls to console.log (e.g. calling it in a loop) could chew up all the memory and die — Why does node/io.js hang when printing to the console inside a loop? #1741, Silly program or memory leak? #2970, Strange memory leaks #3171, memory leaks of console.log #18013.
2. console.log has different behavior while printing to a terminal and being redirected to a file. — Why does node/io.js hang when printing to the console inside a loop? #1741 (comment).
3. Output is sometimes truncated — A libuv upgrade in Node v6 breaks the popular david package #6297, there were other ones as far as I remember.
4. The behaviour seems to differ across platforms.

As I understand it — the output has an implicit write buffer (as it's non-blocking) of unlimited size.

One approach to fixing this would be to:

1. Introduce an explicit cyclic write buffer.
2. Make writes to that cyclic buffer blocking.
3. Make writes from the buffer to the actual output non blocking.
4. When the cyclic buffer reaches it's maximum size (e.g. 10 MiB) — block further writes to the buffer until a corresponding part of it is freed.
5. On (normal) exit, make sure the buffer is flushed.

For almost all cases, except for the ones that are currently broken, this would behave as a non-blocking buffer (because writes to the buffer are considerably faster than writes from the buffer to file/terminal).

For cases when the data is being piped to the output too quickly and when the output file/terminal does not manage to output it at the same rate — the write would turn into a blocking operation. It would also be blocking at the exit until all the data is written.

Another approach would be to monitor (and limit) the size of data that is contained in the implicit buffer coming from the async queue, and make the operations block when that limit is reached.

[ChALkeR]

process.stdout._handle.setBlocking(true); as proposed in #1741 (comment) and #6297 (comment) would considerably hurt the performance. Exposing that isn't a good solution to any of this.

[kzc]

process.stdout._handle.setBlocking(true);
would considerably hurt the performance

Developers are a smart bunch. With the proposed setBlocking API they can decide for themselves if they want stdout/stderr to block on writes. The default could still remain non-blocking. There's no harm in giving people the option. Having CLI applications block on writes for stdout/stderr is quite desirable. This is not unlike all the Sync calls in the fs module. In a high-performance server scenario you don't have to use them, but they are there if you need them.

[ChALkeR]

There's no harm in giving people the option.

There is, every option comes at a cost from both sides. That option would need to be supported, and it would actually just toggle the behaviour between two states that are far from being ideal in many cases.

As for the option here, the said cyclic buffer maximum size could perhaps be configured using a command-line argument, at the buffer size 0 turning in into a fully blocking API.

[kzc]

toggle the behaviour between two states that are far from being ideal in many cases

The proposed API process.stdout._handle.setBlocking(Boolean) could be documented to only be guaranteed to work before any output is sent to the stream.

On UNIX it is a trivial system call on a fd. It's pretty much the same thing on Windows as far as I recall.

Edit: See #6297 (comment)

[mscdex]

The proposed API process.stdout._handle.setBlocking(Boolean) could be documented to only be guaranteed to work before any output is sent to the stream.

We should not be documenting underscore-prefixed properties. This would need to be a separate, true public API.

[sindresorhus]

In CLI tools, I don't want to have stdout/stderr be blocking though, rather I want to be able to explicitly exit the process and have stdout/stderr flushed before exiting. Has there been any discussion about introducing a way to explicitly exit the process gracefully? Meaning flushing the stdout/stderr and then exit. Maybe something like a process.gracefulExit() method? In ES2015 modules, we can no longer return in the top-scope, so short-circuiting will then be effectively impossible, without a nesting mess.

[kzc]

The exit module is a drop in replacement for process.exit() that effectively waits for stdout/stderr to be drained. Other streams can optionally be drained as well. Something like it really ought to rolled into node core with a different name.

Edit: upon further analysis, the exit module is no longer recommended.

[Qix-]

I think there's a slight discrepancy in interpretation here.

Are we talking about console blocking, or console buffering? Albeit similar, these are two different considerations.

Further, I'm not entirely sure what the problem is you're trying to fix, @ChALkeR. Could you elaborate on a problem that is currently being faced? I'm not groking what it is that's broken here.

---

One thing to note is that on most systems (usually UNIX systems) stdout is buffered to begin with, whereas stderr is usually not (or sometimes just line buffered).

However, an explicit fflush(stdout) or fflush(stderr) is not necessary upon a process closing because these buffers are handled by the system and are flushed when main() returns. If the buffer grows too large during execution, the system automatically flushes it.

This is why the following program still outputs while the loop is running, and on most terminal emulators the same reason why it looks like it's 'jittery'.

#include <stdio.h>

int main(void) {
    for (int i = 10000000; i > 0; i--) {
        printf("This is a relatively not-short line of text. :) %d\n", i);
    }
    return 0;
}

---

Blocking, on the other hand, doesn't make much sense. What do you want to block? Are you suggesting we fflush(stdout) every time we console.log()? Because if so, that is a terrible idea.

Developers are a smart bunch. With the proposed setBlocking API they can decide for themselves if they want stdout/stderr to block on writes.

I respectfully disagree. There is absolutely no point in blocking on stdout/stderr. It's very rarely done in the native world and when it is it's usually for concurrency problems (two FILE* handles being written to by two threads that cannot otherwise be locked; i.e. TTY descriptors). Let the system handle it.

The above program without fflush(stdout) on every iteration produced the following average:

real 6.32s
sys  0.54
405 involuntary context switches 

and with fflush(stdout) on each iteration:

real 7.48s
sys  0.56s
1607  involuntary context switches

Thats 18.3% of a time increase on this microbenchmark alone. I'm in the 'time is relative and isn't a great indicator of actual performance' camp, so I included the involuntary context switch count (voluntary context switches would be the same between the two runs, obviously).

The results show that fflush(stdout) upon every message encouraged the system to switch out of the process 1202 more times - that's 296% higher than ye olde system buffering.

Further, the feature that was requested by @sindresorhus is part of how executables are formed - at least on UNIX systems, all file descriptors that are buffered are flushed prior to/during close()-ing or fclose()-ing, flushing prior to a process exiting is a needless step and exposing a new function for it would be completely unnecessary.

---

I'm still lost as to what is trying to be fixed here... I wouldn't mind seeing a process.stdout.flush() method, but it'd be abused in upstream dependencies and I'd find myself writing some pretty annoyed issues in that case. I've never run into a problem with Node's use of standard I/O...

Also, just to reiterate - please don't even start with a .setBlocking() method. That's asking for trouble. The second a dependency makes standard I/O blocking then everything else is affected and then I have to answer to my technical lead about why my simple little program is switching out of context a bazillion times when it didn't before.

[ChALkeR]

@Qix-

Further, I'm not entirely sure what the problem is you're trying to fix, @ChALkeR. Could you elaborate on a problem that is currently being faced? I'm not groking what it is that's broken here.

See the issues I linked above.

Blocking, on the other hand, doesn't make much sense. What do you want to block? Are you suggesting we fflush(stdout) every time we console.log()? Because if so, that is a terrible idea.

No, of course I don't. console.log is not the same as printf in your example.
It's an async operation. I don't propose fflush, I propose to somehow make sure that we don't fill up the async queue with console.logs.