@@ -361,8 +361,24 @@ static const u8 besteffort[] = {
static const u8 normal_order[] = {0, 1, 2, 3, 4, 5, 6, 7};
static const u8 bulk_order[] = {1, 0, 2, 3};
+/* There is a big difference in timing between the accurate values placed in the
+ * cache and the approximations given by a single Newton step for small count
+ * values, particularly when stepping from count 1 to 2 or vice versa. Hence,
+ * these values are calculated using eight Newton steps, using the
+ * implementation below. Above 16, a single Newton step gives sufficient
+ * accuracy in either direction, given the precision stored.
+ *
+ * The magnitude of the error when stepping up to count 2 is such as to give the
+ * value that *should* have been produced at count 4.
+ */
+
#define REC_INV_SQRT_CACHE (16)
-static u32 cobalt_rec_inv_sqrt_cache[REC_INV_SQRT_CACHE] = {0};
+static const u32 inv_sqrt_cache[REC_INV_SQRT_CACHE] = {
+ ~0, ~0, 3037000500, 2479700525,
+ 2147483647, 1920767767, 1753413056, 1623345051,
+ 1518500250, 1431655765, 1358187914, 1294981364,
+ 1239850263, 1191209601, 1147878294, 1108955788
+};
/* http://en.wikipedia.org/wiki/Methods_of_computing_square_roots
* new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
@@ -388,47 +404,14 @@ static void cobalt_newton_step(struct cobalt_vars *vars)
static void cobalt_invsqrt(struct cobalt_vars *vars)
{
if (vars->count < REC_INV_SQRT_CACHE)
- vars->rec_inv_sqrt = cobalt_rec_inv_sqrt_cache[vars->count];
+ vars->rec_inv_sqrt = inv_sqrt_cache[vars->count];
else
cobalt_newton_step(vars);
}
-/* There is a big difference in timing between the accurate values placed in
- * the cache and the approximations given by a single Newton step for small
- * count values, particularly when stepping from count 1 to 2 or vice versa.
- * Above 16, a single Newton step gives sufficient accuracy in either
- * direction, given the precision stored.
- *
- * The magnitude of the error when stepping up to count 2 is such as to give
- * the value that *should* have been produced at count 4.
- */
-
-static void cobalt_cache_init(void)
-{
- struct cobalt_vars v;
-
- memset(&v, 0, sizeof(v));
- v.rec_inv_sqrt = ~0U;
- cobalt_rec_inv_sqrt_cache[0] = v.rec_inv_sqrt;
-
- for (v.count = 1; v.count < REC_INV_SQRT_CACHE; v.count++) {
- cobalt_newton_step(&v);
- cobalt_newton_step(&v);
- cobalt_newton_step(&v);
- cobalt_newton_step(&v);
-
- cobalt_rec_inv_sqrt_cache[v.count] = v.rec_inv_sqrt;
- }
-}
-
static void cobalt_vars_init(struct cobalt_vars *vars)
{
memset(vars, 0, sizeof(*vars));
-
- if (!cobalt_rec_inv_sqrt_cache[0]) {
- cobalt_cache_init();
- cobalt_rec_inv_sqrt_cache[0] = ~0;
- }
}
/* CoDel control_law is t + interval/sqrt(count)