1 files changed, 35 insertions, 12 deletions

diff --git a/directio_paging_speed.c b/directio_paging_speed.c
index b0a01d3..9dd6598 100644
--- a/directio_paging_speed.c
+++ b/directio_paging_speed.c
@@ -1,19 +1,34 @@
+/**
+ * Copyright 2022 Joshua Bakita
+ * This program clocks how long it takes to read, and write, a 1GiB buffer via
+ * Linux direct I/O.
+ *
+ * More precisely, this program clocks:
+ *   write(random_buffer);
+ *   free(random_buffer);
+ * for writing data via direct I/O, and
+ *   malloc(big_buffer);
+ *   read(big_buffer);
+ * for reading in data via direct I/O. `random_buffer` is a preinitialized 1GiB
+ * buffer of random non-zero bytes.
+ */
 #define _GNU_SOURCE
 
-#include <sys/mman.h>
-#include <sys/types.h>
-#include <sys/stat.h>
 #include <fcntl.h>
-#include <stdio.h>
 #include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 #include <time.h>
 #include <unistd.h>
-#include <stdlib.h>
 
 #define GiB 1024l*1024l*1024l
 #define s2ns(s) ((s)*1000l*1000l*1000l)
 #define ns2us(ns) ((ns)/1000l)
 #define PAGED_FILE "/dev/nvme0n1"
+#define CLEAR_PAGECACHE_DENTRIES_INODES "3"
 int max(int x, int y) {return x > y ? x : y;} 
 
 // Original function from copy_only.cu
@@ -31,16 +46,24 @@ uint64_t count_zero(char* buf, uint64_t buf_len) {
         return num_zeros;
 }
 
+// Subtract first parameter from second parameter. Return as nanoseconds.
 long time_diff_ns(struct timespec start, struct timespec stop) {
         return (s2ns(stop.tv_sec) + stop.tv_nsec) - (s2ns(start.tv_sec) + start.tv_nsec);
 }
 
 int main(int argc, char **argv) {
         struct timespec out_start, out_stop, in_start, in_stop;
-        int iters = 1;
-        int res;
-        if (argc > 1)
-                iters = atoi(argv[1]);
+        int iters, res;
+
+        if (argc != 2 || argv[1][0] == '-') {
+                fprintf(stderr, "Usage: %s <number of iterations>\n", argv[0]);
+                return 1;
+        }
+        iters = atoi(argv[1]);
+
+        // If output is redirected, add comment with source details
+        if (!isatty(fileno(stdout)))
+                fprintf(stdout, "# Generated by '%s %s'\n", argv[0], argv[1]);
 
         // Needed to allow page cache clearing between iterations
         // Note: Shouldn't be needed with O_DIRECT, but include it just in case
@@ -49,8 +72,8 @@ int main(int argc, char **argv) {
                 perror("Unable to open /proc/sys/vm/drop_caches");
                 return 1;
         }
-        char clear_cmd = '3';
 
+        // Print table header. One read, one write sample per following row
         printf("out (us)\tin (us)\n");
         for (int i = 0; i < iters; i++) {
                 char *mem_in, *mem_out;
@@ -60,7 +83,7 @@ int main(int argc, char **argv) {
                         return 1;
                 }
                 // Clear page cache
-                write(clear_fd, &clear_cmd, 1);
+                write(clear_fd, CLEAR_PAGECACHE_DENTRIES_INODES, 1);
                 // Allocate and fill a buffer with random data
                 // Aligned malloc(GiB) basicially
                 res = posix_memalign((void**)&mem_in, 4096, GiB);
@@ -81,7 +104,7 @@ int main(int argc, char **argv) {
                 }
                 
                 sleep(1); // Supposedly some other work would happen here
-                write(clear_fd, &clear_cmd, 1); // Just in case O_DIRECT misbehaves
+                write(clear_fd, CLEAR_PAGECACHE_DENTRIES_INODES, 1); // Just in case O_DIRECT misbehaves
                 res = lseek(fd, 0, SEEK_SET); // Reposition offset
                 if (res == -1) {
                         perror("Unable to seek to offset 0 in " PAGED_FILE);