This sets up a randomly permuted single linked list in numademo and walks it.
Advantage over random is that it has a very simple inner loop (four instructions),
but is guaranteed to break the prefetcher.
---
numademo.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 49 insertions(+)
Index: numactl-2.0.0/numademo.c
===================================================================
--- numactl-2.0.0.orig/numademo.c
+++ numactl-2.0.0/numademo.c
@@ -45,6 +45,7 @@ enum test {
BACKWARD,
STREAM,
RANDOM2,
+ PTRCHASE,
} thistest;
char *delim = " ";
@@ -61,6 +62,7 @@ char *testname[] = {
#ifdef HAVE_MT
"random2",
#endif
+ "ptrchase",
NULL,
};
@@ -94,6 +96,39 @@ void do_stream(char *name, unsigned char
}
#endif
+/* Set up a randomly distributed list to fool prefetchers */
+union node {
+ union node *next;
+ struct {
+ unsigned nexti;
+ unsigned val;
+ };
+};
+
+static int cmp_node(const void *ap, const void *bp)
+{
+ union node *a = (union node *)ap;
+ union node *b = (union node *)bp;
+ return a->val - b->val;
+}
+
+void **ptrchase_init(unsigned char *mem)
+{
+ long i;
+ union node *nodes = (union node *)mem;
+ long nmemb = msize / sizeof(union node);
+ srand(1234);
+ for (i = 0; i < nmemb; i++) {
+ nodes[i].val = rand();
+ nodes[i].nexti = i + 1;
+ }
+ qsort(nodes, nmemb, sizeof(union node), cmp_node);
+ for (i = 0; i < nmemb; i++) {
+ union node *n = &nodes[i];
+ n->next = n->nexti >= nmemb ? NULL : &nodes[n->nexti];
+ }
+ return (void **)nodes;
+}
static inline unsigned long long timerfold(struct timeval *tv)
{
@@ -122,6 +157,19 @@ void memtest(char *name, unsigned char *
sum = 0;
for (i = 0; i < LOOPS; i++) {
switch (thistest) {
+ case PTRCHASE:
+ {
+ void **ptr;
+ ptr = ptrchase_init(mem);
+ gettimeofday(&start,NULL);
+ while (*ptr)
+ ptr = (void **)*ptr;
+ gettimeofday(&end,NULL);
+ /* Side effect to trick the optimizer */
+ *ptr = "bla";
+ break;
+ }
+
case MEMSET:
gettimeofday(&start,NULL);
memset(mem, 0xff, msize);
@@ -408,6 +456,7 @@ int main(int ac, char **av)
#ifdef HAVE_STREAM_LIB
test(STREAM);
#endif
+ test(PTRCHASE);
} else {
int k;
for (k = 2; k < ac; k++) {
--
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