The existing code did a 64 bit divide directly, which won't work on
32 bit platforms. This is what I am testing, it uses math similar to
TCP CUBIC to do a quad root. It seemed more efficient to just do
one operation rather than two square roots.
---------
diff --git a/net/ipv4/tcp_compound.c b/net/ipv4/tcp_compound.c
index 01048e2..74c26a0 100644
--- a/net/ipv4/tcp_compound.c
+++ b/net/ipv4/tcp_compound.c
@@ -52,8 +52,6 @@
#define TCP_COMPOUND_ALPHA 3U
#define TCP_COMPOUND_BETA 1U
-#define TCP_COMPOUND_KAPPA_POW 3
-#define TCP_COMPOUND_KAPPA_NSQRT 2
#define TCP_COMPOUND_GAMMA 30
#define TCP_COMPOUND_ZETA 1
@@ -156,6 +154,58 @@ static void tcp_compound_state(struct so
vegas_disable(sk);
}
+
+/* 64bit divisor, dividend and result. dynamic precision */
+static inline u64 div64_64(u64 dividend, u64 divisor)
+{
+ u32 d = divisor;
+
+ if (divisor > 0xffffffffULL) {
+ unsigned int shift = fls(divisor >> 32);
+
+ d = divisor >> shift;
+ dividend >>= shift;
+ }
+
+ /* avoid 64 bit division if possible */
+ if (dividend >> 32)
+ do_div(dividend, d);
+ else
+ dividend = (u32) dividend / d;
+
+ return dividend;
+}
+
+/* calculate the quartic root of "a" using Newton-Raphson */
+static u32 qroot(u64 a)
+{
+ u32 x, x1;
+
+ /* Initial estimate is based on:
+ * qrt(x) = exp(log(x) / 4)
+ */
+ x = 1u << (fls64(a) >> 2);
+
+ /*
+ * Iteration based on:
+ * 3
+ * x = ( 3 * x + a / x ) / 4
+ * k+1 k k
+ */
+ do {
+ u64 x3 = x;
+
+ x1 = x;
+ x3 *= x;
+ x3 *= x;
+
+ x = (3 * x + (u32) div64_64(a, x3)) / 4;
+ } while (abs(x1 - x) > 1);
+
+ return x;
+}
+
+
/*
* If the connection is idle and we are restarting,
* then we don't want to do any Vegas calculations
@@ -307,29 +357,23 @@ static void tcp_compound_cong_avoid(stru
dwnd = vegas->dwnd;
if (diff < (TCP_COMPOUND_GAMMA << V_PARAM_SHIFT)) {
- u32 i, j, x, x2;
- u64 v;
-
- v = 1;
-
- for (i = 0; i < TCP_COMPOUND_KAPPA_POW; i++)
- v *= old_wnd;
-
- for (i = 0; i < TCP_COMPOUND_KAPPA_NSQRT; i++) {
- x = 1;
- for (j = 0; j < 200; j++) {
- x2 = (x + v / x) / 2;
-
- if (x2 == x || !x2)
- break;
-
- x = x2;
- }
- v = x;
- }
+ u64 win3;
- x = (u32) v >> TCP_COMPOUND_ALPHA;
+ /*
+ * The TCP Compound paper describes the choice
+ * of "k" determines the agressiveness,
+ * ie. slope of the response function.
+ *
+ * For same value as HSTCP would be 0.8
+ * but for computaional reasons, both the
+ * original authors and this implementation
+ * use 0.75.
+ */
+ win3 = old_wnd;
+ win3 *= old_wnd;
+ win3 *= old_wnd;
+ x = qroot(win3) >> TCP_COMPOUND_ALPHA;
if (x > 1)
dwnd = x - 1;
else
-
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