pagingtest: switch to advising random accesses

Switch to using madvise(MADV_RANDOM) and posix_fadvise(POSIX_FADV_RANDOM)
to disable caching as opposed to reading the file backwards, which still
went through some caching.

Measured a 80% drop in worst case performance after switching.

Change-Id: I03115dfb94ae7dfc58e0e187af3936c4e371817f

2 files changed, 18 insertions, 7 deletions

diff --git a/tests/pagingtest/pageinout_test.c b/tests/pagingtest/pageinout_test.c
index b9b20de6..eaa116c1 100644
--- a/tests/pagingtest/pageinout_test.c
+++ b/tests/pagingtest/pageinout_test.c
@@ -13,7 +13,7 @@ int pageinout_test(int test_runs, unsigned long long file_size) {
     char tmpname[] = "pageinoutXXXXXX";
     unsigned char *vec;
     int i;
-    long long j;
+    unsigned long long j;
     volatile char *buf;
     int ret = -1;
     int rc;
@@ -43,10 +43,17 @@ int pageinout_test(int test_runs, unsigned long long file_size) {
         goto err;
     }
 
+    //madvise and fadvise as random to prevent prefetching
+    rc = madvise((void *)buf, file_size, MADV_RANDOM) ||
+           posix_fadvise(fd, 0, file_size, POSIX_FADV_RANDOM);
+    if (rc) {
+        goto err;
+    }
+
     for (i = 0; i < test_runs; i++) {
         gettimeofday(&begin_time, NULL);
-        //Read backwards to prevent mmap prefetching
-        for (j = ((file_size - 1) & ~(pagesize - 1)); j >= 0; j -= pagesize) {
+        //read every page into the page cache
+        for (j = 0; j < file_size; j += pagesize) {
             buf[j];
         }
         gettimeofday(&end_time, NULL);
diff --git a/tests/pagingtest/thrashing_test.c b/tests/pagingtest/thrashing_test.c
index 7ecd3ade..b0193d85 100644
--- a/tests/pagingtest/thrashing_test.c
+++ b/tests/pagingtest/thrashing_test.c
@@ -14,7 +14,7 @@ int thrashing_test(int test_runs) {
     int fds[4] = {-1, -1, -1, -1};
     char tmpnames[4][17] = { "thrashing1XXXXXX", "thrashing2XXXXXX", "thrashing3XXXXXX", "thrashing4XXXXXX" };
     volatile char *bufs[4] = {0};
-    long long k;
+    unsigned long long k;
     int ret = -1;
     struct timeval begin_time, end_time, elapsed_time, total_time;
     unsigned long long filesize;
@@ -45,14 +45,18 @@ int thrashing_test(int test_runs) {
             fprintf(stderr, "Failed to mmap file: %s\n", strerror(errno));
             goto err;
         }
+        //madvise and fadvise as random to prevent prefetching
+        ret = madvise((void *)bufs[i], filesize, MADV_RANDOM) ||
+               posix_fadvise(fds[i], 0, filesize, POSIX_FADV_RANDOM);
+        if (ret) {
+            goto err;
+        }
     }
 
     for (int i = 0; i < test_runs; i++) {
         for (size_t j = 0; j < ARRAY_SIZE(fds); j++) {
             gettimeofday(&begin_time, NULL);
-            //Unfortunately when under memory pressure, fadvise and madvise stop working...
-            //Read backwards to prevent mmap prefetching
-            for (k = ((filesize - 1) & ~(pagesize - 1)); k >= 0; k -= pagesize) {
+            for (k = 0; k < filesize; k += pagesize) {
                 bufs[j][k];
             }
             gettimeofday(&end_time, NULL);