bench: optimize io.c benchmark

Use static buffers. Most clock ticks were spent in malloc() and free() instead
of read() and write().

Fix measurements. Really fast runs would result in bogus results like:

  Wrote 1048576000 bytes in -0.731630s using 8192 byte buffers: -1366.811093mB/s

benchmark/io.c

@@ -2,41 +2,47 @@
  * gcc -o iotest io.c
  */
 
+#include <assert.h>
 #include <unistd.h>
 #include <string.h>
 #include <fcntl.h>
 #include <sys/time.h>
 #include <assert.h>
 #include <stdlib.h>
+#include <stdint.h>
 #include <stdio.h>
  
-int tsize = 1000 * 1048576;
-const char *path = "/tmp/wt.dat";
+static int c = 0;
+static int tsize = 1000 * 1048576;
+static const char path[] = "/tmp/wt.dat";
+static char buf[65536];
 
-int c = 0;
+static uint64_t now(void) {
+  struct timeval tv;
 
-char* bufit(size_t l)
-{
-  char *p = malloc(l);
-  memset(p, '!', l);
-  return p;
+  if (gettimeofday(&tv, NULL))
+    abort();
+
+  return tv.tv_sec * 1000000ULL + tv.tv_usec;
 }
 
-void writetest(int size, size_t bsize)
+static void writetest(int size, size_t bsize)
 {
   int i;
-  char *buf = bufit(bsize);
-  struct timeval start, end;
+  uint64_t start, end;
   double elapsed;
   double mbps;
 
+  assert(bsize <= sizeof buf);
+
   int fd = open(path, O_CREAT|O_WRONLY, 0644);
   if (fd < 0) {
     perror("open failed");
     exit(254);
   }
 
-  assert(0 ==  gettimeofday(&start, NULL));
+  start = now();
+
   for (i = 0; i < size; i += bsize) {
     int rv = write(fd, buf, bsize);
     if (c++ % 2000 == 0) fprintf(stderr, ".");
@@ -45,35 +51,40 @@ void writetest(int size, size_t bsize)
       exit(254);
     }
   }
-#ifdef __linux__
-  fdatasync(fd);
-#else
-  fsync(fd);
-#endif
   close(fd);
-  assert(0 == gettimeofday(&end, NULL));
-  elapsed = (end.tv_sec - start.tv_sec) + ((double)(end.tv_usec - start.tv_usec))/100000.;
-  mbps = ((tsize/elapsed)) / 1048576;
-  fprintf(stderr, "\nWrote %d bytes in %03fs using %ld byte buffers: %03fmB/s\n", size, elapsed, bsize, mbps);
 
-  free(buf);
+#ifndef NSYNC
+# ifdef __linux__
+  fdatasync(fd);
+# else
+  fsync(fd);
+# endif
+#endif /* SYNC */
+
+  end = now();
+  elapsed = (end - start) / 1e6;
+  mbps = ((tsize/elapsed)) / 1048576;
+
+  fprintf(stderr, "\nWrote %d bytes in %03fs using %ld byte buffers: %03fmB/s\n", size, elapsed, bsize, mbps);
 }
 
 void readtest(int size, size_t bsize)
 {
   int i;
-  char *buf = bufit(bsize);
-  struct timeval start, end;
+  uint64_t start, end;
   double elapsed;
   double mbps;
 
+  assert(bsize <= sizeof buf);
+
   int fd = open(path, O_RDONLY, 0644);
   if (fd < 0) {
     perror("open failed");
     exit(254);
   }
 
-  assert(0 == gettimeofday(&start, NULL));
+  start = now();
+
   for (i = 0; i < size; i += bsize) {
     int rv = read(fd, buf, bsize);
     if (rv < 0) {
@@ -82,12 +93,12 @@ void readtest(int size, size_t bsize)
     }
   }
   close(fd);
-  assert(0 == gettimeofday(&end, NULL));
-  elapsed = (end.tv_sec - start.tv_sec) + ((double)(end.tv_usec - start.tv_usec))/100000.;
-  mbps = ((tsize/elapsed)) / 1048576;
-  fprintf(stderr, "Read %d bytes in %03fs using %ld byte buffers: %03fmB/s\n", size, elapsed, bsize, mbps);
 
-  free(buf);
+  end = now();
+  elapsed = (end - start) / 1e6;
+  mbps = ((tsize/elapsed)) / 1048576;
+
+  fprintf(stderr, "Read %d bytes in %03fs using %ld byte buffers: %03fmB/s\n", size, elapsed, bsize, mbps);
 }
 
 void cleanup() {