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Commit 006042d7 authored by Gerrit Renker's avatar Gerrit Renker Committed by Arnaldo Carvalho de Melo
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[DCCP] tfrc: Identify TFRC table limits and simplify code 



This
 * adds documentation about the lowest resolution that is possible within
   the bounds of the current lookup table
 * defines a constant TFRC_SMALLEST_P which defines this resolution
 * issues a warning if a given value of p is below resolution
 * combines two previously adjacent if-blocks of nearly identical
   structure into one

This patch does not change the algorithm as such.

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 8d0086ad

net/dccp/ccids/lib/tfrc_equation.c

+18 −9
Original line number Diff line number Diff line
@@ -19,6 +19,7 @@ 


#define TFRC_CALC_X_ARRSIZE 500

#define TFRC_CALC_X_SPLIT   50000	/* 0.05 * 1000000, details below */

#define TFRC_SMALLEST_P	    (TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE)



/*

  TFRC TCP Reno Throughput Equation Lookup Table for f(p)
@@ -68,7 +69,9 @@ 
  granularity for the practically more relevant case of small values of p (up to

  5%), the second column is used; the first one ranges up to 100%.  This split

  corresponds to the value of q = TFRC_CALC_X_SPLIT. At the same time this also

  determines the smallest resolution.

  determines the smallest resolution possible with this lookup table:



    TFRC_SMALLEST_P   =  TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE



  The entire table is generated by:

    for(i=0; i < TFRC_CALC_X_ARRSIZE; i++) {
@@ -79,7 +82,7 @@ 
  With the given configuration, we have, with M = TFRC_CALC_X_ARRSIZE-1,

    lookup[0][0]  =  g(1000000/(M+1)) 		 =  1000000 * f(0.2%)

    lookup[M][0]  =  g(1000000)			 =  1000000 * f(100%)

    lookup[0][1]  =  g(TFRC_CALC_X_SPLIT/(M+1))	 =  1000000 * f(0.01%)

    lookup[0][1]  =  g(TFRC_SMALLEST_P)		  = 1000000 * f(0.01%)

    lookup[M][1]  =  g(TFRC_CALC_X_SPLIT)	 =  1000000 * f(5%)



  In summary, the two columns represent f(p) for the following ranges:
@@ -616,16 +619,22 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p) 
		return ~0U;

 	}



	if (p < TFRC_CALC_X_SPLIT) 		      /* 0      <= p <  0.05  */

		index = (p / (TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE)) - 1;

	else		 			      /* 0.05   <= p <= 1.00  */

		index = (p / (1000000 / TFRC_CALC_X_ARRSIZE)) - 1;

	if (p <= TFRC_CALC_X_SPLIT) 		{     /* 0.0000 < p <= 0.05   */

		if (p < TFRC_SMALLEST_P) {	      /* 0.0000 < p <  0.0001 */

			DCCP_WARN("Value of p (%d) below resolution. "

				  "Substituting %d\n", p, TFRC_SMALLEST_P);

			index = 0;

		} else 				      /* 0.0001 <= p <= 0.05  */

			index =  p/TFRC_SMALLEST_P - 1;



	if (p >= TFRC_CALC_X_SPLIT)

		f = tfrc_calc_x_lookup[index][0];

	else

 		f = tfrc_calc_x_lookup[index][1];



	} else {	 			      /* 0.05   <  p <= 1.00  */

		index = p/(1000000/TFRC_CALC_X_ARRSIZE) - 1;



		f = tfrc_calc_x_lookup[index][0];

	}



	/* The following computes X = s/(R*f(p)) in bytes per second. Since f(p)

	 * and R are both scaled by 1000000, we need to multiply by 1000000^2.

	 * ==> DO NOT alter this unless you test against overflow on 32 bit   */