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Codel is a new AQM algorithm and RED replacement designed by Kathie Nichols and Van Jacobson, and published in ACM queue: http://queue.acm.org/detail.cfm?id=2209336 Codel stands for "Controlled Delay", and needs no knobs in the general case, twiddled, for optimum results. It aims for 5ms of delay, at most, when in use. Additionally, fq_codel (by eric dumazet) builds on codel to provide fair queuing superior to what could be had with SFQ, and drop behavior saner than RED, BLUE, or choke. These patches are backported from net-next and are known to work on Linux 3.3.4 and later. Includes updates to codel for better portability and speed git-svn-id: svn://svn.openwrt.org/openwrt/trunk@31756 3c298f89-4303-0410-b956-a3cf2f4a3e73
86 lines
2.9 KiB
Diff
86 lines
2.9 KiB
Diff
From 6ff272c9ad65eda219cd975b9da2dbc31cc812ee Mon Sep 17 00:00:00 2001
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From: Eric Dumazet <eric.dumazet@gmail.com>
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Date: Sat, 12 May 2012 21:23:23 +0000
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Subject: [PATCH 46/50] codel: use u16 field instead of 31bits for
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rec_inv_sqrt
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David pointed out gcc might generate poor code with 31bit fields.
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Using u16 is more than enough and permits a better code output.
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Also make the code intent more readable using constants, fixed point arithmetic
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not being trivial for everybody.
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Suggested-by: David Miller <davem@davemloft.net>
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Signed-off-by: Eric Dumazet <edumazet@google.com>
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Signed-off-by: David S. Miller <davem@davemloft.net>
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---
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include/net/codel.h | 25 +++++++++++++++----------
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1 file changed, 15 insertions(+), 10 deletions(-)
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--- a/include/net/codel.h
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+++ b/include/net/codel.h
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@@ -133,13 +133,17 @@ struct codel_params {
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struct codel_vars {
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u32 count;
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u32 lastcount;
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- bool dropping:1;
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- u32 rec_inv_sqrt:31;
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+ bool dropping;
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+ u16 rec_inv_sqrt;
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codel_time_t first_above_time;
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codel_time_t drop_next;
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codel_time_t ldelay;
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};
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+#define REC_INV_SQRT_BITS (8 * sizeof(u16)) /* or sizeof_in_bits(rec_inv_sqrt) */
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+/* needed shift to get a Q0.32 number from rec_inv_sqrt */
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+#define REC_INV_SQRT_SHIFT (32 - REC_INV_SQRT_BITS)
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+
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/**
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* struct codel_stats - contains codel shared variables and stats
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* @maxpacket: largest packet we've seen so far
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@@ -173,17 +177,18 @@ static void codel_stats_init(struct code
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* http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
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* new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
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*
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- * Here, invsqrt is a fixed point number (< 1.0), 31bit mantissa)
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+ * Here, invsqrt is a fixed point number (< 1.0), 32bit mantissa, aka Q0.32
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*/
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static void codel_Newton_step(struct codel_vars *vars)
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{
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- u32 invsqrt = vars->rec_inv_sqrt;
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- u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 31;
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- u64 val = (3LL << 31) - ((u64)vars->count * invsqrt2);
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+ u32 invsqrt = ((u32)vars->rec_inv_sqrt) << REC_INV_SQRT_SHIFT;
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+ u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 32;
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+ u64 val = (3LL << 32) - ((u64)vars->count * invsqrt2);
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- val = (val * invsqrt) >> 32;
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+ val >>= 2; /* avoid overflow in following multiply */
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+ val = (val * invsqrt) >> (32 - 2 + 1);
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- vars->rec_inv_sqrt = val;
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+ vars->rec_inv_sqrt = val >> REC_INV_SQRT_SHIFT;
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}
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/*
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@@ -195,7 +200,7 @@ static codel_time_t codel_control_law(co
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codel_time_t interval,
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u32 rec_inv_sqrt)
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{
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- return t + reciprocal_divide(interval, rec_inv_sqrt << 1);
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+ return t + reciprocal_divide(interval, rec_inv_sqrt << REC_INV_SQRT_SHIFT);
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}
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@@ -326,7 +331,7 @@ static struct sk_buff *codel_dequeue(str
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codel_Newton_step(vars);
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} else {
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vars->count = 1;
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- vars->rec_inv_sqrt = 0x7fffffff;
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+ vars->rec_inv_sqrt = ~0U >> REC_INV_SQRT_SHIFT;
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}
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vars->lastcount = vars->count;
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vars->drop_next = codel_control_law(now, params->interval,
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