Improve random pattern without norandommap

Using the random map, random workloads have a natural tendency to
get less random at the end of the run, due to failures to find a
new unused random offset.

Improve the logic to avoid this problem.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/io_u.c b/io_u.c
index e892ec759c761de338f9e5789f74c13a3bbfb45c..c824d7ecce4ef8246c9304c5f4a7ef77fa458a0d 100644 (file)
--- a/io_u.c
+++ b/io_u.c
@@ -109,20 +109,40 @@ static int get_next_rand_offset(struct thread_data *td, struct fio_file *f,
                        *b = 0;
                else
                        *b = ((max_blocks - 1) * r / (unsigned long long) (RAND_MAX+1.0));
                        *b = 0;
                else
                        *b = ((max_blocks - 1) * r / (unsigned long long) (RAND_MAX+1.0));

+               /*
+                * if we are not maintaining a random map, we are done.
+                */

                if (td->o.norandommap)
                if (td->o.norandommap)

-                       break;
+                       return 0;
+
+               /*
+                * calculate map offset and chec if it's free
+                */

                rb = *b + (f->file_offset / td->o.min_bs[ddir]);
                rb = *b + (f->file_offset / td->o.min_bs[ddir]);

-               loops--;
-       } while (!random_map_free(td, f, rb) && loops);
+               if (random_map_free(td, f, rb))
+                       return 0;
+
+       } while (--loops);

 
        /*
 
        /*

-        * if we failed to retrieve a truly random offset within
-        * the loops assigned, see if there are free ones left at all
+        * we get here, if we didn't suceed in looking up a block. generate
+        * a random start offset into the filemap, and find the first free
+        * block from there.

         */
         */

-       if (!loops && get_next_free_block(td, f, b))
-               return 1;
+       loops = 10;
+       do {
+               f->last_free_lookup = (f->num_maps - 1) * (r / (RAND_MAX+1.0));
+               if (!get_next_free_block(td, f, b))
+                       return 0;

 
 

-       return 0;
+               r = os_random_long(&td->random_state);
+       } while (--loops);
+
+       /*
+        * that didn't work either, try exhaustive search from the start
+        */
+       f->last_free_lookup = 0;
+       return get_next_free_block(td, f, b);

 }
 
 /*
 }
 
 /*