[DCCP]: Binary search for reverse TFRC lookup
This replaces the linear search algorithm for reverse lookup with
binary search.
It has the advantage of better scalability: O(log2(N)) instead of O(N).
This means that the average number of iterations is reduced from 250
(linear search if each value appears equally likely) down to at most 9.
Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
---
net/dccp/ccids/lib/tfrc_equation.c | 38 +++++++++++++++++++++++--------------
1 file changed, 24 insertions(+), 14 deletions(-)
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -574,6 +574,21 @@ static const u32 tfrc_calc_x_lookup[TFRC
{ 243315981, 271305 }
};
+/* return largest index i such that fval <= lookup[i][small] */
+static inline u32 tfrc_binsearch(u32 fval, u8 small)
+{
+ u32 try, low = 0, high = TFRC_CALC_X_ARRSIZE - 1;
+
+ while(low < high) {
+ try = (low + high)/2;
+ if (fval <= tfrc_calc_x_lookup[try][small])
+ high = try;
+ else
+ low = try + 1;
+ }
+ return high;
+}
+
/**
* tfrc_calc_x - Calculate the send rate as per section 3.1 of RFC3448
*
@@ -636,8 +651,7 @@ EXPORT_SYMBOL_GPL(tfrc_calc_x);
*/
u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
{
- int ctr = 0;
- int small;
+ int index;
if (fvalue == 0) /* f(p) = 0 whenever p = 0 */
return 0;
@@ -652,18 +666,14 @@ u32 tfrc_calc_x_reverse_lookup(u32 fvalu
return 1000000;
}
- if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1])
- small = 1;
- else
- small = 0;
-
- while (fvalue > tfrc_calc_x_lookup[ctr][small])
- ctr++;
-
- if (small)
- return (ctr + 1) * TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE;
- else
- return (ctr + 1) * 1000000/TFRC_CALC_X_ARRSIZE;
+ if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1]) {
+ index = tfrc_binsearch(fvalue, 1);
+ return (index + 1) * TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE;
+ }
+
+ /* else ... it must be in the coarse-grained column */
+ index = tfrc_binsearch(fvalue, 0);
+ return (index + 1) * 1000000/TFRC_CALC_X_ARRSIZE;
}
EXPORT_SYMBOL_GPL(tfrc_calc_x_reverse_lookup);
-
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