(per-project or overall) if there are no pending tasks.
This is needed when there are coproc (i.e. CUDA) jobs;
CPUs may be idle because pending jobs are waiting for active jobs
to release coprocs.
In this situation the CPU idleness should not be counted as shortfall;
otherwise (if there are only coproc jobs) there will always be a shortfall,
and the client will fetch infinite work.
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