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workers: implement --max-worker-threads command line option

Rodrigo Muino Tomonari requested to merge github/fork/jasnell/max-worker-count into master

There are two commits here:

  1. Adds a simple CPUInfo utility to make using uv_cpu_info just a tad easier
  2. Implements the --max-worker-threads command line flag..

From the second commit message:

    Creating too many active worker threads at one time can
    lead to significant performance degradation of the entire
    Node.js process. This adds a worker thread counter that
    will cause a warning to be emitted if exceeded. Workers
    can still be created beyond the limit, however. The warning
    is similar in spirit to the too many event handlers warning
    emitted by EventEmitter.

    By default, the limit is one less than four times the total
    number of CPUs available calculated at system start. The
    `--max-worker-threads` command-line option can be set to
    set a non-default value. The option is permitted in
    `NODE_OPTIONS` and must be positive number greater than
    zero.

    The counter and the option are per-process in order to
    account for Workers that create their own Workers.

    The warning will be emitted once each time the limit
    is exceeded, so may be emitted more than once per process.
    That is, if the limit is 2, and 5 workers are created, only
    a single warning will be emitted. If the number of active
    workers falls back below 2 and is subsequently exceeded
    again, the warning will be emitted again.

/cc @addaleax

Checklist
  • make -j4 test (UNIX), or vcbuild test (Windows) passes
  • tests and/or benchmarks are included
  • documentation is changed or added
  • commit message follows commit guidelines

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