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Commit 2bbf29ac authored by Gerrit Renker's avatar Gerrit Renker Committed by Arnaldo Carvalho de Melo
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[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: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: default avatarIan McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@mandriva.com>
parent 44158306

net/dccp/ccids/lib/tfrc_equation.c

+24 −14
Original line number Diff line number Diff line
@@ -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 (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;

 	}

 

	if (small)

		return (ctr + 1) * TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE;

	else

		return (ctr + 1) * 1000000 / 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);