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tools/diskbench.c reports page-cache throughput on Linux, not disk #22

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tools/diskbench.c reports page-cache throughput on Linux, not disk #22

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19 changes: 16 additions & 3 deletions tools/diskbench.c
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Original file line number Diff line number Diff line change
Expand Up @@ -28,11 +28,24 @@ static double now(void) {
return t.tv_sec + t.tv_usec / 1e6;
}

/* Linux has no F_NOCACHE; the page cache is bypassed with O_DIRECT at open
* time instead, which is what the engine itself does (see model.c). Without it
* this benchmark reports page-cache throughput on Linux — e.g. 42 GB/s
* sequential on a drive whose link tops out at 3.9 GB/s. Buffers here are
* already posix_memalign'd to 4096 and the record size is a 4096 multiple, so
* O_DIRECT's alignment requirements are met. */
#if defined(__linux__) && defined(O_DIRECT)
#define DIO_FLAG O_DIRECT
#else
#define DIO_FLAG 0
#endif

static void nocache(int fd) {
#ifdef __APPLE__
fcntl(fd, F_NOCACHE, 1);
fcntl(fd, F_RDAHEAD, 0);
#endif
(void)fd;
}

static const char *g_path;
Expand All @@ -43,7 +56,7 @@ typedef struct { int id, nthreads; double bytes; } targ;

static void *rand_reader(void *p) {
targ *a = (targ *)p;
int fd = open(g_path, O_RDONLY);
int fd = open(g_path, O_RDONLY | DIO_FLAG);
if (fd < 0) { perror("open"); return NULL; }
nocache(fd);
void *buf = NULL;
Expand Down Expand Up @@ -75,7 +88,7 @@ int main(int argc, char **argv) {
printf("file %.1f GB, record %.1f MB, path %s\n", file_gb, rec_mb, g_path);

/* 1. sequential write */
int fd = open(g_path, O_WRONLY | O_CREAT | O_TRUNC, 0644);
int fd = open(g_path, O_WRONLY | O_CREAT | O_TRUNC | DIO_FLAG, 0644);
if (fd < 0) { perror("open write"); return 1; }
nocache(fd);
void *buf;
Expand All @@ -92,7 +105,7 @@ int main(int argc, char **argv) {
printf("seq write : %6.2f GB/s\n", written / dt / (1u << 30));

/* 2. sequential read, cache-bypassed */
fd = open(g_path, O_RDONLY); nocache(fd);
fd = open(g_path, O_RDONLY | DIO_FLAG); nocache(fd);
t0 = now(); size_t rd = 0; ssize_t r;
while ((r = read(fd, buf, g_rec)) > 0) rd += r;
dt = now() - t0; close(fd);
Expand Down