Re: [v3 PATCH 0/9] rhashtable: Multiple rehashing
From: Herbert Xu <herbert@gondor.apana.org.au>
Date: 2015-03-24 02:38:01
Subsystem:
library code, rhashtable, the rest · Maintainers:
Andrew Morton, Thomas Graf, Herbert Xu, Linus Torvalds
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 +1100quoted
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>
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;
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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