Static GC_parallel 

pub unsafe static mut GC_parallel: c_int

GC is parallelized for performance on multiprocessors. Set to a nonzero value when client calls GC_start_mark_threads() directly or starts the first non-main thread, provided the collector is built with PARALLEL_MARK macro defined, and either GC_MARKERS (or GC_NPROCS) environment variable is set to a value bigger than 1, or multiple cores (processors) are available, or the client calls GC_set_markers_count() before the collector initialization. After setting, GC_parallel value is equal to the number of marker threads minus one (i.e. the number of existing parallel marker threads excluding the initiating one).