Debugger is very slow and uses too much memory when large lists are around

[DartBot]

This issue was originally filed by podivilov@google.com

---

1. Create VM App:

import 'dart:scalarlist';

void main() {
  var list = new Int8List(16 * 1000 * 1000);
  print(list.length);
  print('ok');
}

1. Set two breakpoints on both prints.
2. Run application
3. Debugger hangs for 50 seconds
4. When debugger is finally paused on first print, hit continue
5. "Exhausted heap space, trying to allocate 268435488 bytes." error message appears in console and debugger hangs again for another 50 seconds. After pausing it prints the error message once again.

Without breakpoints this same application runs to completion in less than a second.

This issue makes the debugger mostly unusable if you have large list in your program, so setting priority to high.

[devoncarew]

Pavel, I added some code to the command-line debugger to display large lists in slices instead of the whole list at once. This was committed last week; can you confirm that the version you saw this behavior was trying to display slices?

[DartBot]

This comment was originally written by podivilov@google.com

---

You are right. I was using an older version. The latest build looks much better, thanks!

Still, each debugger step takes ~5 seconds and I get lots of "Exhausted heap space, trying to allocate 268435488 bytes." messages in console, which perhaps indicate that something is going wrong.

Overall, the Editor is still quite slow when debugging the sample.

[DartBot]

This comment was originally written by podivilov@google.com

---

Removed Priority-High label.
Added Priority-Medium label.

[DartBot]

This comment was originally written by @mhausner

---

I think what's happening is that the wire protocol is calling the toString() method of the list variable to transmit a textual representation of the value. As a side effect, a huge buffer for the C string gets allocated. The wire protocol needs to be smarter when creating the textual value.

---

Added Accepted label.

[DartBot]

This comment was originally written by @mhausner

---

I have a change that improves the situation. For this program:

void main() {
  var list = new List(1000);
  list[1] = new A();
  list[2] = 4.9;
  list[3] = new List(1000);
  list[4] = "The Fifth Element";
  list[5] = { "foo": 700 };
  print(list.length);
  print('ok');
}

class A {
  toString() => throw "foo!";
}

The wire protocol now submits this info when breaking at the print(ok) statement:

Isolate 7114 paused on breakpoint
Stack trace:
0 main (file:///Users/hausner/tmp/d.dart:10) (lib 15)
  list = (list, id 0, len 1000) [null, #ERROR, 4.9, [...], "The Fifth Element", {foo: 700}, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, ...]

Before this change, the debugger choked on this.

https://codereview.chromium.org/12033039/

[DartBot]

This comment was originally written by @mhausner

---

Added another change (r17517) to restrict the length of the text that gets submitted. This problem should be solved now.

---

Added Fixed label.