disable multithreading for small workloads

interface/lapack/trtri.c

@@ -127,6 +127,9 @@ int NAME(char *UPLO, char *DIAG, blasint *N, FLOAT *a, blasint *ldA, blasint *In
 #endif
 
 #ifdef SMP
+if (args.n <= 150)
+  args.nthreads = 1;
+else
   args.nthreads = num_cpu_avail(4);
 
   if (args.nthreads == 1) {