When reading memory in order, HW prefetching optimizations will
interfere with measuring how caches and memory are being accessed. This
adds noise into the results.
Change the fill_buf reading loop to not use an obvious in-order
access using multiply by a prime and modulo.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
---
tools/testing/selftests/resctrl/fill_buf.c | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)
diff --git a/tools/testing/selftests/resctrl/fill_buf.c b/tools/testing/selftests/resctrl/fill_buf.c
index 7e0d3a1ea555..049a520498a9 100644
--- a/tools/testing/selftests/resctrl/fill_buf.c
+++ b/tools/testing/selftests/resctrl/fill_buf.c
@@ -88,14 +88,17 @@ static void *malloc_and_init_memory(size_t s)
static int fill_one_span_read(unsigned char *start_ptr, unsigned char *end_ptr)
{
- unsigned char sum, *p;
-
+ unsigned int size = (end_ptr - start_ptr) / (CL_SIZE / 2);
+ unsigned int count = size;
+ unsigned char sum;
+
+ /*
+ * Read the buffer in an order that is unexpected by HW prefetching
+ * optimizations to prevent them interfering with the caching pattern.
+ */
sum = 0;
- p = start_ptr;
- while (p < end_ptr) {
- sum += *p;
- p += (CL_SIZE / 2);
- }
+ while (count--)
+ sum += start_ptr[((count * 59) % size) * CL_SIZE / 2];
return sum;
}
--
2.30.2
