Attempt to tame _hemv threading #1820

interface/zhemv.c

@@ -195,7 +195,12 @@ void CNAME(enum CBLAS_ORDER order, enum CBLAS_UPLO Uplo, blasint n, void *VALPHA
   buffer = (FLOAT *)blas_memory_alloc(1);
 
 #ifdef SMP
-  nthreads = num_cpu_avail(2);
+  // see graph in issue #1820 for explanation and room for improvement
+  if (n<362) {
+	  nthreads = 1 ;
+  } else {
+  	  nthreads = num_cpu_avail(2);
+  };
 
   if (nthreads == 1) {
 #endif