lfsr: fix verification and spin bugs

Changes:

1. Verification now works properly and reports for which value it fails
2. Fix a mishandling of spin incrementation which led to multiple
calculation of the same values

Signed-off-by: Alex Pyrgiotis <apyrgio@grnet.gr>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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.
- * 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 repeat;
-       unsigned int spin;
+       unsigned int spin = fl->spin;
 
        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 {
+               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);
+check: ;
        } 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);
-       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++) {
@@ -198,6 +208,7 @@ int prepare_spin(struct fio_lfsr *fl, unsigned int spin)
                        break;
                }
        }
+       fl->cached_cycle_length = fl->cycle_length;
 
        return 0;
 }

diff --git a/lib/lfsr.h b/lib/lfsr.h
index bc16af966af4a350a25bd2119dc551b09cc6ed2e..187abf2fc0f2438562f8ae1d4b82b0e506d2d0e3 100644 (file)
--- a/lib/lfsr.h
+++ b/lib/lfsr.h
@@ -18,6 +18,7 @@ struct fio_lfsr {
        uint64_t max_val;
        uint64_t num_vals;
        uint64_t cycle_length;
+       uint64_t cached_cycle_length;
        unsigned int spin;
 };
 

diff --git a/t/lfsr-test.c b/t/lfsr-test.c
index 193a7f9f77ea5929936b986be179d15111ffb88c..d371087cf42a85b3c8ded574fb2fc88bcfd5cddf 100644 (file)
--- a/t/lfsr-test.c
+++ b/t/lfsr-test.c
@@ -94,13 +94,15 @@ int main(int argc, char *argv[])
        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
        fprintf(stderr, "finished.\n");
 
+
        /* Check if all expected numbers within range have been calculated */
        r = 0;
        if (verify) {
                fprintf(stderr, "Verifying results... ");
                for (i = 0; i < numbers; i++) {
-                       if (*(uint8_t *)(v + 1) != 1) {
-                               fprintf(stderr, "failed.\n");
+                       if (*(uint8_t *)(v + i) != 1) {
+                               fprintf(stderr, "failed (%lu = %d).\n",
+                                               i, *(uint8_t *)(v + i));
                                r = 1;
                                break;
                        }