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authorGerrit Renker <gerrit@erg.abdn.ac.uk>2006-12-03 17:53:27 +0100
committerArnaldo Carvalho de Melo <acme@mandriva.com>2006-12-03 17:53:27 +0100
commit2bbf29acd8f7adcf161de7e5d891b4095687a59f (patch)
tree955b15fa3bcdee1f879949d8639c5e9d5b82e84e
parent[DCCP] ccid3: Deprecate TFRC_SMALLEST_P (diff)
downloadlinux-2bbf29acd8f7adcf161de7e5d891b4095687a59f.tar.xz
linux-2bbf29acd8f7adcf161de7e5d891b4095687a59f.zip
[DCCP] tfrc: 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@erg.abdn.ac.uk> Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz> Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
-rw-r--r--net/dccp/ccids/lib/tfrc_equation.c38
1 files changed, 24 insertions, 14 deletions
diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
index 0a4a3d2feba5..ddac2c511e2f 100644
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -594,6 +594,21 @@ static const u32 tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE][2] = {
{ 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
*
@@ -656,8 +671,7 @@ 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;
@@ -672,18 +686,14 @@ u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
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);