Non-Dart Tool invocations aren't throttled

[gspencergoog]

If I swap the snippets tool in Flutter's repo with the following "slow_command"script:

#!/bin/bash

# Runs slowly because it never gets snapshotted.
dart /home/gspencer/code/flutter/dev/snippets/lib/main.dart "$@"

by using this config:

# This file is used by dartdoc when generating API documentation for Flutter.
dartdoc:
  # Before you can run dartdoc, the snippets tool needs to have a snapshot built.
  # The dev/tools/dartdoc.dart script does this automatically.
  tools:
    snippet:
      command: ["slow_command", "--type=snippet"]
      description: "Creates sample code documentation output from embedded documentation samples."
    sample:
      command: ["slow_command", "--type=sample"]
      description: "Creates full application sample code documentation output from embedded documentation samples."
    dartpad:
      command: ["slow_command", "--type=sample", "--dartpad"]
      description: "Creates full application sample code documentation output from embedded documentation samples and displays it in an embedded DartPad."
  errors:
    # Default errors of dartdoc:
    - duplicate-file
    - invalid-parameter
    - no-defining-library-found
    - tool-error
    - unresolved-export
    # Warnings that are elevated to errors:
    - ambiguous-doc-reference
    - ambiguous-reexport
    - broken-link
    - category-order-gives-missing-package-name
    - deprecated
    - ignored-canonical-for
    - missing-from-search-index
    - no-canonical-found
    - no-documentable-libraries
    - no-library-level-docs
    - not-implemented
    - orphaned-file
    - reexported-private-api-across-packages
    # - unknown-directive  # Disabled due to https://github.com/dart-lang/dartdoc/issues/2353
    - unknown-file
    - unknown-macro
    - unresolved-doc-reference

Then it seems to run out of control, and I can get my load average up to >1000:

This doesn't happen if I keep the regular flutter config that runs a snapshotted version of the same script.

I'm trying to convert the snippet tool over to be run using pub global run from a package, and the lack of throttling is preventing that. I know that there is throttling code, but I just don't think it's working for non-dart tools.

[srawlins]

@jcollins-g and I had both experienced some weird non-throttling behavior on-and-off, but couldn't really figure out what was going on. I think you've provided some valuable info here. I'd love to get this issue fixed.

[gspencergoog]

I also tried reverting the dartdoc_options.yaml, and adding a 10-second pause into the snippet tool, and it does appear to throttle to 4 processes when it uses the snapshotting (i.e. when it's a Dart tool and not a shell script). I did notice an initial spike in that run when it had at least 50 running simultaneously, but it quickly calmed down to 4, so maybe there's some initialization pass that isn't throttled or something?

Incidentally, I think it would be good to look at the number of CPUs available instead of just hard coding four: my 72-core machine would be a lot faster to run dartdoc in that case (as would our CI when configured with more CPUs). I believe that Platform.numberOfProcessors has that information.

[gspencergoog]

@srawlins I have a fix for this. I'll post a PR this afternoon.