snow: per-cpu rand_r() seed state

Snow was already threaded, but used a global seed with rand_r() meaning
the CPUs were hammering on the same address.  There wasn't any locking or
atomics, as it isn't terribly critical when generating white noise if the
seed access is racy.  But the writes still caused cache lines to ping-pong.

This commit gives a ~15.5% speedup in my measurements on an i7-2640M.

Note without the padded union, using just an array of ints, zero gain
is realized.  I used a padding of 256 just to have some headroom, x86
is 64 but other CPUs vary, POWER9 is 128 for example.

1 files changed, 38 insertions, 9 deletions

diff --git a/src/modules/snow/snow.c b/src/modules/snow/snow.c
index 8968e47..3e6331a 100644
--- a/src/modules/snow/snow.c
+++ b/src/modules/snow/snow.c
@@ -9,13 +9,37 @@
 
 /* This implements white noise / snow just using rand() */
 
-/* To avoid the contention around rand() I'm just using rand_r()
- * in an entirely racy fashion with a single seed from the threaded
- * render_fragment().  It should really be per-cpu but the module
- * api doesn't currently send down a cpu identifier to the render
- * function, so TODO in the future add that.
- */
-static int	snow_seed;
+typedef union snow_seed_t {
+	char	__padding[256];		/* prevent seeds sharing a cache-line */
+	int	seed;
+} snow_seed_t;
+
+typedef struct snow_context_t {
+	int		foo;		/* make the compiler happy */
+	snow_seed_t	seeds[];
+} snow_context_t;
+
+
+static void * snow_create_context(unsigned n_cpus)
+{
+	snow_context_t	*ctxt;
+
+	ctxt = calloc(1, sizeof(snow_context_t) + n_cpus * sizeof(snow_seed_t));
+	if (!ctxt)
+		return NULL;
+
+	for (unsigned i = 0; i < n_cpus; i++)
+		ctxt->seeds[i].seed = rand();
+
+	return ctxt;
+}
+
+
+static void snow_destroy_context(void *context)
+{
+	free(context);
+}
+
 
 static int snow_fragmenter(void *context, const fb_fragment_t *fragment, unsigned number, fb_fragment_t *res_fragment)
 {
@@ -31,9 +55,12 @@ static void snow_prepare_frame(void *context, unsigned n_cpus, fb_fragment_t *fr
 
 static void snow_render_fragment(void *context, unsigned cpu, fb_fragment_t *fragment)
 {
+	snow_context_t	*ctxt = context;
+	int		*seed = &ctxt->seeds[cpu].seed;
+
 	for (unsigned y = fragment->y; y < fragment->y + fragment->height; y++) {
 		for (unsigned x = fragment->x; x < fragment->x + fragment->width; x++) {
-			uint32_t	pixel = rand_r(&snow_seed) % 256;
+			uint32_t	pixel = rand_r(seed) % 256;
 
 			fb_fragment_put_pixel_unchecked(fragment, x, y, pixel << 16 | pixel << 8 | pixel);
 		}
@@ -42,10 +69,12 @@ static void snow_render_fragment(void *context, unsigned cpu, fb_fragment_t *fra
 
 
 rototiller_module_t	snow_module = {
+	.create_context = snow_create_context,
+	.destroy_context = snow_destroy_context,
 	.prepare_frame = snow_prepare_frame,
 	.render_fragment = snow_render_fragment,
 	.name = "snow",
-	.description = "TV snow / white noise",
+	.description = "TV snow / white noise (threaded)",
 	.author = "Vito Caputo <vcaputo@pengaru.com>",
 	.license = "GPLv2",
 };