Introduce a (crude) threshold to multithreading

Author: martin-frbg

interface/trmv.c

@@ -219,7 +219,10 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_UPLO Uplo,
 
 #ifdef SMP
   nthreads = num_cpu_avail(2);
-
+  
+  if (n < 50 ) nthreads = 1;
+  if (nthreads > 2 && n < 500) nthreads = 2;
+	
   if (nthreads == 1) {
 #endif