Messages in this thread |  | | Subject | Re: ip_conntrack_hash() problem | From | Martin Wilck <> | Date | 04 Sep 2002 15:40:51 +0200 |
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Just to make my previous statement clearer:
I think there's nothing wrong with a power-of-2 hashsize, as long as the hash function contains no implicit or explicit multipliers that are also powers of two. In general, multipliers should not have a greatest common divisor (GCD) larger than 1 with the hash size. Unfortunately, in the current implementation, ntohl() creates an implicit multiplier of 2^16 for the port numbers (on little-endian machines).
Martin
PS: For the sake of that, the patch also changed the multiplier for the source port from 2 to 7, assuming that it's relatively unlikely to have a hash size that is a multiple of 7, and knowing that multiplying by 7 is cheap. Instead of 7, 31 or 127 also seem good candidates that are even more unlikely to be divisors of the hash size.
I recommend to printk() a warning if the hash size turns out to have a GCD >1 with multiple of any multiplier in the hash function.
-- Martin Wilck Phone: +49 5251 8 15113 Fujitsu Siemens Computers Fax: +49 5251 8 20409 Heinz-Nixdorf-Ring 1 mailto:Martin.Wilck@Fujitsu-Siemens.com D-33106 Paderborn http://www.fujitsu-siemens.com/primergy
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