diff mbox

[v3,0/9] rhashtable: Multiple rehashing

Message ID 20150324023729.GA7973@gondor.apana.org.au
State Accepted, archived
Delegated to: David Miller
Headers show

Commit Message

Herbert Xu March 24, 2015, 2:37 a.m. UTC 
On Mon, Mar 23, 2015 at 10:09:13PM -0400, David Miller wrote:
> From: Herbert Xu <herbert@gondor.apana.org.au>
> Date: Tue, 24 Mar 2015 00:49:55 +1100
> 
> > This series introduces multiple rehashing.
> 
> This looks great, nice work.  All applied to net-next.

Thanks!

> Could you please add a comment next to the "16" from patch
> #9 explaining why that value was choosen and the invariants
> that influence it?

Sure.

---8<---
rhashtable: Add comment on choice of elasticity value

This patch adds a comment on the choice of the value 16 as the
maximum chain length before we force a rehash.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Comments

David Miller March 24, 2015, 6:57 p.m. UTC | #1 
From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Tue, 24 Mar 2015 13:37:30 +1100

> rhashtable: Add comment on choice of elasticity value
> 
> This patch adds a comment on the choice of the value 16 as the
> maximum chain length before we force a rehash.
> 
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Thanks for following up on this, applied.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
diff mbox

Patch

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index e96ad1a..8514f7c 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -736,6 +736,18 @@  int rhashtable_init(struct rhashtable *ht,
 
 	ht->p.min_size = max(ht->p.min_size, HASH_MIN_SIZE);
 
+	/* The maximum (not average) chain length grows with the
+	 * size of the hash table, at a rate of (log N)/(log log N).
+	 * The value of 16 is selected so that even if the hash
+	 * table grew to 2^32 you would not expect the maximum
+	 * chain length to exceed it unless we are under attack
+	 * (or extremely unlucky).
+	 *
+	 * As this limit is only to detect attacks, we don't need
+	 * to set it to a lower value as you'd need the chain
+	 * length to vastly exceed 16 to have any real effect
+	 * on the system.
+	 */
 	if (!params->insecure_elasticity)
 		ht->elasticity = 16;