- reduce the number of calls to notify_one() and notify_all().
notify_one() is not only called when running a new job, and
notify_all() only when a job finishes.
- don't hold the condition lock while calling notify_*(), as it is not
strictly needed, and because notify_*() can be very slow, there can
be a lot of contention on this lock as a result; blocking the whole
jobsystem thread pool.
- add a new version of run() that takes an opaque thread id that can
be retrieved from a job's execute function; this is especially
intended to be used by parallel_for(); it's just a more efficient
version of run() that avoids a hashmap lookup.
Overall these change yield a significant performance boost:
- running + waiting a job: +200%
- running many jobs: +150%
- running many jobs in parallel: +50%
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