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[DCCP] tfrc: Binary search for reverse TFRC lookup

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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>
This commit is contained in:
Gerrit Renker 2006-12-03 14:53:27 -02:00 committed by Arnaldo Carvalho de Melo
parent 44158306d7
commit 2bbf29acd8
1 changed files with 24 additions and 14 deletions
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38
net/dccp/ccids/lib/tfrc_equation.c
View file

@ -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);