From 3ae30cd6b9407cea58aa9d32c3f8e8916153cf8e Mon Sep 17 00:00:00 2001
From: Jerome Robert <jeromerobert@gmx.com>
Date: Fri, 15 Jan 2016 17:12:04 +0100
Subject: [PATCH] Disable multi-threading for small matrices in [z]ger

Ref #731
---
 interface/ger.c  | 7 +++++--
 interface/zger.c | 6 +++++-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/interface/ger.c b/interface/ger.c
index 2d19e3bc0..b2f803dec 100644
--- a/interface/ger.c
+++ b/interface/ger.c
@@ -174,8 +174,11 @@ void CNAME(enum CBLAS_ORDER order,
   STACK_ALLOC(m, FLOAT, buffer);
 
 #ifdef SMPTEST
-  nthreads = num_cpu_avail(2);
-
+  // Threshold chosen so that speed-up is > 1 on a Xeon E5-2630
+  if(1L * m * n > 24L * GEMM_MULTITHREAD_THRESHOLD)
+    nthreads = num_cpu_avail(2);
+  else
+    nthreads = 1;
 
   if (nthreads == 1) {
 #endif
diff --git a/interface/zger.c b/interface/zger.c
index f46a462e2..5bcbfc09b 100644
--- a/interface/zger.c
+++ b/interface/zger.c
@@ -213,7 +213,11 @@ void CNAME(enum CBLAS_ORDER order,
   buffer = (FLOAT *)blas_memory_alloc(1);
 
 #ifdef SMPTEST
-  nthreads = num_cpu_avail(2);
+  // Threshold chosen so that speed-up is > 1 on a Xeon E5-2630
+  if(1L * m * n > 3L * sizeof(FLOAT) * GEMM_MULTITHREAD_THRESHOLD)
+    nthreads = num_cpu_avail(2);
+  else
+    nthreads = 1;
 
   if (nthreads == 1) {
 #endif