[IPV4]: fib_trie root-node expansion

The patch below introduces special thresholds to keep root node in the trie 
large. This gives a flatter tree at the cost of a modest memory increase.
Overall it seems to be gain and this was also proposed by one the authors 
of the paper in recent a seminar.

Main table after loading 123 k routes.

	Aver depth:     3.30
	Max depth:      9
        Root-node size  12 bits
        Total size: 4044  kB

With the patch:
	Aver depth:     2.78
	Max depth:      8
        Root-node size  15 bits
        Total size: 4150  kB

An increase of 8-10% was seen in forwading performance for an rDoS attack. 

Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 21 insertions, 2 deletions

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 50c0519cd70d..0093ea08c7f5 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -286,6 +286,8 @@ static inline void check_tnode(const struct tnode *tn)
 
 static int halve_threshold = 25;
 static int inflate_threshold = 50;
+static int halve_threshold_root = 15;
+static int inflate_threshold_root = 25; 
 
 
 static void __alias_free_mem(struct rcu_head *head)
@@ -449,6 +451,8 @@ static struct node *resize(struct trie *t, struct tnode *tn)
         int i;
         int err = 0;
         struct tnode *old_tn;
+        int inflate_threshold_use;
+        int halve_threshold_use;
 
         if (!tn)
                 return NULL;
@@ -541,10 +545,17 @@ static struct node *resize(struct trie *t, struct tnode *tn)
 
         check_tnode(tn);
 
+        /* Keep root node larger  */
+
+        if(!tn->parent)
+                inflate_threshold_use = inflate_threshold_root;
+        else 
+                inflate_threshold_use = inflate_threshold;
+
         err = 0;
         while ((tn->full_children > 0 &&
                50 * (tn->full_children + tnode_child_length(tn) - tn->empty_children) >=
-                                inflate_threshold * tnode_child_length(tn))) {
+                                inflate_threshold_use * tnode_child_length(tn))) {
 
                 old_tn = tn;
                 tn = inflate(t, tn);
@@ -564,10 +575,18 @@ static struct node *resize(struct trie *t, struct tnode *tn)
          * node is above threshold.
          */
 
+
+        /* Keep root node larger  */
+
+        if(!tn->parent)
+                halve_threshold_use = halve_threshold_root;
+        else 
+                halve_threshold_use = halve_threshold;
+
         err = 0;
         while (tn->bits > 1 &&
                100 * (tnode_child_length(tn) - tn->empty_children) <
-               halve_threshold * tnode_child_length(tn)) {
+               halve_threshold_use * tnode_child_length(tn)) {
 
                 old_tn = tn;
                 tn = halve(t, tn);