lfsr: fix verification and spin bugs

[fio.git] / lib / lfsr.c

diff --git a/lib/lfsr.c b/lib/lfsr.c
index 4c15c6256f9a1d41ab95a1bfdb0d8e59fb92796a..b10ba7a7e1b3cfc07609b53f6c68317fa14cb53e 100644 (file)
--- a/lib/lfsr.c
+++ b/lib/lfsr.c
@@ -112,26 +112,36 @@ static inline void __lfsr_next(struct fio_lfsr *fl, unsigned int spin)
  * lfsr_next does the following:
  *
  * a. Return if the number of max values has been exceeded.
  * lfsr_next does the following:
  *
  * a. Return if the number of max values has been exceeded.

- * b. Check if the next iteration(s) produce a cycle (due to spin) and add "1"
- *    where necessary.
- * c. Calculate the next value and return.
+ * b. Check if we have a spin value that produces a repeating subsequence.
+ *    This is previously calculated in `prepare_spin` and cycle_length should
+ *    be > 0. If we do have such a spin:
+ *
+ *    i. Decrement the calculated cycle.
+ *    ii. If it reaches zero, add "+1" to the spin and reset the cycle_length
+ *        (we have it cached in the struct fio_lfsr)
+ *
+ *    In either case, continue with the calculation of the next value.
+ * c. Check if the calculated value exceeds the desirable range. In this case,
+ *    go back to b, else return.

  */
 int lfsr_next(struct fio_lfsr *fl, uint64_t *off, uint64_t last)
 {
  */
 int lfsr_next(struct fio_lfsr *fl, uint64_t *off, uint64_t last)
 {

-       int repeat;
-       unsigned int spin;
+       unsigned int spin = fl->spin;

 
        if (fl->num_vals++ > fl->max_val)
                return 1;
 
 
        if (fl->num_vals++ > fl->max_val)
                return 1;
 

-       repeat = fl->num_vals % fl->cycle_length;
-       if (repeat == 0)
-               spin = fl->spin + 1;
-       else
-               spin = fl->spin;
-

        do {
        do {

+               if (fl->cycle_length) {
+                       fl->cycle_length--;
+                       if (!fl->cycle_length) {
+                               __lfsr_next(fl, fl->spin + 1);
+                               fl->cycle_length = fl->cached_cycle_length;
+                               goto check;
+                       }
+               }

                __lfsr_next(fl, spin);
                __lfsr_next(fl, spin);

+check: ;

        } while (fl->last_val > fl->max_val);
 
        *off = fl->last_val;
        } while (fl->last_val > fl->max_val);
 
        *off = fl->last_val;


@@ -189,7 +199,7 @@ int prepare_spin(struct fio_lfsr *fl, unsigned int spin)
 
        x = max / (spin + 1);
        y = max % (spin + 1);
 
        x = max / (spin + 1);
        y = max % (spin + 1);

-       fl->cycle_length = max; /* This is the expected cycle */
+       fl->cycle_length = 0;   /* No cycle occurs, other than the expected */

        fl->spin = spin;
 
        for (i = 1; i <= spin; i++) {
        fl->spin = spin;
 
        for (i = 1; i <= spin; i++) {


@@ -198,6 +208,7 @@ int prepare_spin(struct fio_lfsr *fl, unsigned int spin)
                        break;
                }
        }
                        break;
                }
        }

+       fl->cached_cycle_length = fl->cycle_length;

 
        return 0;
 }
 
        return 0;
 }