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Date:	Mon, 20 Oct 2008 10:12:51 +0200
From:	Eric Dumazet <>
To:	Neil Horman <>
Cc:	David Miller <>,,,,,,,,, Stephen Hemminger <>
Subject: Re: [PATCH] net: implement emergency route cache rebulds when	gc_elasticity
 is exceeded

Neil Horman a écrit :
> Ok, one more revision. It takes Erics final suggestions into account.  Eric, as
> per your request, I've added your signed off line to it.
> This is a patch to provide on demand route cache rebuilding.  Currently, our
> route cache is rebulid periodically regardless of need.  This introduced
> unneeded periodic latency.  This patch offers a better approach.  Using code
> provided by Eric Dumazet, we compute the standard deviation of the average hash
> bucket chain length while running rt_check_expire.  Should any given chain
> length grow to larger that average plus 4 standard deviations, we trigger an
> emergency hash table rebuild for that net namespace.  This allows for the common
> case in which chains are well behaved and do not grow unevenly to not incur any
> latency at all, while those systems (which may be being maliciously attacked),
> only rebuild when the attack is detected.  This patch take 2 other factors into
> account:
> 1) chains with multiple entries that differ by attributes that do not affect the
> hash value are only counted once, so as not to unduly bias system to rebuilding
> if features like QOS are heavily used
> 2) if rebuilding crosses a certain threshold (which is adjustable via the added
> sysctl in this patch), route caching is disabled entirely for that net
> namespace, since constant rebuilding is less efficient that no caching at all
> Tested successfully by me.
> Regards
> Neil
> Signed-off-by: Neil Horman <>
> Signed-off-by: Eric Dumazet <>

Thanks Neil for your patience, this patch is fine.

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