Reference documentation for deal.II version 9.4.1

Variables  
unsigned int  minimum_parallel_grain_size = 4096 
unsigned int internal::VectorImplementation::minimum_parallel_grain_size = 4096 
If we do computations on vectors in parallel (say, we add two vectors to get a third, and we do the loop over all elements in parallel), then this variable determines the minimum number of elements for which it is profitable to split a range of elements any further to distribute to different threads.
This variable is available as a global writable variable in order to allow the testsuite to also test the parallel case. By default, it is set to several thousand elements, which is a case that the testsuite would not normally encounter. As a consequence, in the testsuite we set it to one – a value that's hugely unprofitable but definitely tests parallel operations.
Definition at line 34 of file parallel.cc.