[gnome-photos/gnome-3-26] gegl: Set the number of threads to match physical CPU cores

[Debarshi Ray <debarshir src gnome org>]

commit d790e9b30b7a704f932c4f718f552cea59cfb143
Author: Debarshi Ray <debarshir gnome org>
Date:   Sun Oct 29 23:33:07 2017 +0100

    gegl: Set the number of threads to match physical CPU cores
    
    Increasing the number of threads beyond the number of physical CPU
    cores doesn't yield any measurable performance improvements. In fact,
    in some cases, a small drop was noted. Presumably because of memory
    bandwidth limits.

 src/photos-gegl.c |   22 +++++++++++++++++++++-
 1 files changed, 21 insertions(+), 1 deletions(-)
---
diff --git a/src/photos-gegl.c b/src/photos-gegl.c
index cac55da..2110561 100644
--- a/src/photos-gegl.c
+++ b/src/photos-gegl.c
@@ -292,7 +292,25 @@ photos_gegl_get_buffer_from_node (GeglNode *node, const Babl *format)
 void
 photos_gegl_init (void)
 {
+  GeglConfig *config;
+  gint threads;
+  guint num_processors;
+
   gegl_init (NULL, NULL);
+
+  num_processors = g_get_num_processors ();
+  g_return_if_fail (num_processors > 0);
+
+  /* The number of threads should match the number of physical CPU
+   * cores, not the number of virtual hyper-threading cores. In the
+   * absence of an API to get the number of physical CPU cores, we
+   * assume that a number higher than one is indicative of
+   * hyper-threading, and hence divide by two.
+   */
+  threads = (gint) (num_processors > 1 ? num_processors / 2 : num_processors);
+
+  config = gegl_config ();
+  g_object_set (config, "threads", threads, NULL);
 }
 
 
@@ -412,10 +430,12 @@ photos_gegl_sanity_check (void)
   GeglConfig *config;
   gboolean ret_val = TRUE;
   gboolean use_opencl;
+  gint threads;
   guint i;
 
   config = gegl_config ();
-  g_object_get (config, "use-opencl", &use_opencl, NULL);
+  g_object_get (config, "threads", &threads, "use-opencl", &use_opencl, NULL);
+  photos_debug (PHOTOS_DEBUG_GEGL, "GEGL: Threads: %d", threads);
   photos_debug (PHOTOS_DEBUG_GEGL, "GEGL: Using OpenCL: %s", use_opencl ? "yes" : "no");
 
   for (i = 0; i < G_N_ELEMENTS (REQUIRED_GEGL_OPS); i++)