Change default concurrency from 4 to available number of cpus

Expected Behavior

It would be nice if concurrency defaulted to os.cpus()

Current Behavior

Concurrency defaults to 4 no matter what.

Possible Solution

Change the DEFAULT_CONCURRENCY variable to be os.cpus(). Alternatively, enable this only with a flag to avoid a breaking change.

Context

This would allow jobs to scale easily with different instance sizes when running in things like EC2. I have some tasks that run sometimes in a 4 core machine and sometimes in a 16 core machine or even larger.

Most local machines have 4 cores anyway, so they would remain unchanged. But it’s hard to maximize concurrency using only the CLI right now.

"run-stuff": "lerna run task --concurrency 8"

If I have a package.json task like above, it’s hard to access the number of available CPUS to set the concurrency level. I need to instead run the lerna task from a node script, which is less convenient. Ideally, it would always maximize concurrency without needing to avoid the CLI. Or at least have that be available with a flag, like --maximize-concurrency.

If this is something you’ll consider, I’m happy to do a PR to add it.

[andrewkincaid]

My apologies! Setting concurrency in lerna.json works.

[worc]

is concurrency still a valid option? it’s not listed in the docs:

https://github.com/lerna/lerna#lernajson