[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: 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>
diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
index ef3233d..0a4a3d2 100644
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -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:
@@ -616,15 +619,21 @@
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][1];
+
+ } else { /* 0.05 < p <= 1.00 */
+ index = p/(1000000/TFRC_CALC_X_ARRSIZE) - 1;
+
f = tfrc_calc_x_lookup[index][0];
- else
- f = tfrc_calc_x_lookup[index][1];
+ }
/* 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.