[Pixman] [PATCH 1/1] Improve L1 and L2 benchmark tests for caches that don't use allocate-on-write

[Ben Avison]

In particular this affects single-core ARMs (e.g. ARM11, Cortex-A8), which
are usually configured this way. For other CPUs, this should only add a
constant time, which will be cancelled out by the EXCLUDE_OVERHEAD runs.

The problems were caused by cachelines becoming permanently evicted from
the cache, because the code that was intended to pull them back in again on
each iteration assumed too long a cache line (for the L1 test) or failed to
read memory beyond the first pixel row (for the L2 test). Also, the reloading
of the source buffer was unnecessary.

These issues were identified by Siarhei in this post:
http://lists.freedesktop.org/archives/pixman/2013-January/002543.html
---
 test/lowlevel-blt-bench.c |   31 +++++++++++++++++++++++++------
 1 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/test/lowlevel-blt-bench.c b/test/lowlevel-blt-bench.c
index 7336fa0..8e80b42 100644
--- a/test/lowlevel-blt-bench.c
+++ b/test/lowlevel-blt-bench.c
@@ -33,6 +33,14 @@
 #define L1CACHE_SIZE (8 * 1024)
 #define L2CACHE_SIZE (128 * 1024)
 
+/* This is applied to both L1 and L2 tests - alternatively, you could
+ * parameterise bench_L or split it into two functions. It could be
+ * read at runtime on some architectures, but it only really matters
+ * that it's a number that's an integer divisor of both cacheline
+ * lengths, and further, it only really matters for caches that don't
+ * do allocate0on-write. */
+#define CACHELINE_LENGTH (32) /* bytes */
+
 #define WIDTH  1920
 #define HEIGHT 1080
 #define BUFSIZE (WIDTH * HEIGHT * 4)
@@ -168,18 +176,29 @@ bench_L  (pixman_op_t              op,
           int                      width,
           int                      lines_count)
 {
-    int64_t      i, j;
+    int64_t      i, j, k;
     int          x = 0;
     int          q = 0;
     volatile int qx;
 
     for (i = 0; i < n; i++)
     {
-	/* touch destination buffer to fetch it into L1 cache */
-	for (j = 0; j < width + 64; j += 16) {
-	    q += dst[j];
-	    q += src[j];
-	}
+        /* For caches without allocate-on-write, we need to force the
+         * destination buffer back into the cache on each iteration,
+         * otherwise if they are evicted during the test, they remain
+         * uncached. This doesn't matter for tests which read the
+         * destination buffer, or for caches that do allocate-on-write,
+         * but in those cases this loop just adds constant time, which
+         * should be successfully cancelled out.
+         */
+        for (j = 0; j < lines_count; j++)
+        {
+            for (k = 0; k < width + 62; k += CACHELINE_LENGTH / sizeof *dst)
+            {
+                q += dst[j * WIDTH + k];
+            }
+            q += dst[j * WIDTH + width + 62];
+        }
 	if (++x >= 64)
 	    x = 0;
 	call_func (func, op, src_img, mask_img, dst_img, x, 0, x, 0, 63 - x, 0, width, lines_count);
-- 
1.7.5.4