baa8ddb6___Compiling___Running_Programs__Description
[Environment Variable] SPINLOOPTIME=n (default = 40) [Description] 
Sets the number of times a user thread will spin-idle when it cannot acquire a lock (e.g., before it begins a parallel loop) When the spin count has been exhausted, the thread will go to sleep waiting for a lock to become available ... unless the YIELDLOOPTIME environment variable is set to a number greater than zero. You want to spin rather than sleep if you are waiting for a previous parallel loop to complete, provided there is not too much sequential work between the parallelized loops. If YIELDLOOPTIME is set, upon exhausting the spin count, the thread issues the yield() system call, gives up the processor, but stays in a runnable state rather than going to sleep. On a quiet system, yielding is preferable to sleeping since reactivating the thread after sleeping is more costly. On a busy system, SPINLOOPTIME should not be set too large, otherwize valuable processor time that could be shared with other jobs is consumed spinning. Some experimentation may be required to reach an optimal condition.
TAMU Supercomputing Facility -- Hydra User Guide: Compiling & Running Programs
Description