[Libguestfs] [libnbd PATCH 1/2] examples: Enhance access patterns of threaded-reads-and-writes

[Eric Blake]

A typical client will probably be interleaving small and large
requests, rather than using uniform requests everywhere. Update the
test to randomly simulate this.

For me, this cuts test runtime from 23.1s to 12.6s.
---
 examples/threaded-reads-and-writes.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/examples/threaded-reads-and-writes.c b/examples/threaded-reads-and-writes.c
index 1f66f2f..6d09cfc 100644
--- a/examples/threaded-reads-and-writes.c
+++ b/examples/threaded-reads-and-writes.c
@@ -46,7 +46,7 @@ static int64_t exportsize;
  */
 #define MAX_IN_FLIGHT 64

-/* The size of reads and writes. */
+/* The size of large reads and writes, must be > 512. */
 #define BUFFER_SIZE (1024*1024)

 /* Number of commands we issue (per thread). */
@@ -188,7 +188,9 @@ start_thread (void *arg)
   uint64_t handles[MAX_IN_FLIGHT];
   size_t in_flight;        /* counts number of requests in flight */
   int dir, r, cmd;
+  size_t size;

+  assert (512 < BUFFER_SIZE);
   buf = malloc (BUFFER_SIZE);
   if (buf == NULL) {
     perror ("malloc");
@@ -237,14 +239,16 @@ start_thread (void *arg)
      * the same buffer for multiple in-flight requests.  It doesn't
      * matter here because we're just trying to write random stuff,
      * but that would be Very Bad in a real application.
+     * Simulate a mix of large and small requests.
      */
     while (i > 0 && in_flight < MAX_IN_FLIGHT) {
-      offset = rand () % (exportsize - BUFFER_SIZE);
+      size = (rand() & 1) ? BUFFER_SIZE : 512;
+      offset = rand () % (exportsize - size);
       cmd = rand () & 1;
       if (cmd == 0)
-        handle = nbd_aio_pwrite (nbd, buf, BUFFER_SIZE, offset, 0);
+        handle = nbd_aio_pwrite (nbd, buf, size, offset, 0);
       else
-        handle = nbd_aio_pread (nbd, buf, BUFFER_SIZE, offset, 0);
+        handle = nbd_aio_pread (nbd, buf, size, offset, 0);
       if (handle == -1) {
         fprintf (stderr, "%s\n", nbd_get_error ());
         goto error;
-- 
2.20.1