feat(global-options): Default concurrency to logical CPU count

[ghost]

Description

The PR makes little changes to set concurrency option to count of logical CPU cores by default.

Motivation and Context

--concurrency option is set to 4 by default what can be not the best value with different count of CPUs.
Setting the option to specific number isn't good options cause the same script can be run on different servers.

Fixes #1929

How Has This Been Tested?

Unfortunately, my computer have 4 logical CPU what was the value before my changes, so for now I didn't test what is effected. I'm going to run lerna tests in virtual environments with different count of CPU cores.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[evocateur]

Overall, I'm concerned that os.cpus().length isn't always the appropriate value for concurrent operations. Not that 4 is much better, it's pretty arbitrary.

[evocateur]

(My concerns aside, I'm in favor of this change)

[bdwain]

Overall, I'm concerned that os.cpus().length isn't always the appropriate value for concurrent operations. Not that 4 is much better, it's pretty arbitrary.

I agree. But when it's not, my guess is they probably have a hard coded number in mind anyway, so they can just set the concurrency if they want.

If people wanted to use half of the cpus, or all but one, they'd need a node script to do that. But that's the case today today, and if it was that popular of a request, there could be options such as --use-half-cpus or --use-all-but-one-cpu. I doubt they'd be necessary though.

[fyodorvi]

FYI, it seems that this feature caused us some grief in CircleCI where running a command across 30+ packages would randomly fail on one of the packages with:

lerna ERR! yarn run build stderr:
error Command failed with signal "SIGKILL".

We fixed it by explicitly adding --concurrency 2 (2 is the default number of core CircleCI gives you).

[sidharthachatterjee]

CircleCI doesn't report number of cores correctly so this fails on it without concurrency being added explicitly 😞

[evocateur]

Sounds like a Circle CI bug to me.