netdev - Re: Extensible hashing and RCU

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Date:	Sun, 18 Feb 2007 14:04:24 -0800
From:	"Michael K. Edwards" <medwards.linux@...il.com>
To:	"Eric Dumazet" <dada1@...mosbay.com>
Cc:	"Evgeniy Polyakov" <johnpol@....mipt.ru>, akepner@....com,
	linux@...izon.com, davem@...emloft.net, netdev@...r.kernel.org,
	bcrl@...ux.intel.com
Subject: Re: Extensible hashing and RCU

On 2/18/07, Michael K. Edwards <medwards.linux@...il.com> wrote:
> ...  Much less vulnerable to cache eviction DDoS
> than a hash, because the hot connections get rotated up into non-leaf
> layers and get traversed enough to keep them in the LRU set.

Let me enlarge on this a bit.  I used to work for a company that built
a custom firewall/VPN ASIC with all sorts of special sauce in it,
mostly focused on dealing with DDoS.  Some really smart guys, some
really good technology, I hope they grab the brass ring someday.  On
the scale they were dealing with, there's only one thing to do about
DDoS: bend over and take it.  Provision enough memory bandwidth to
cold-cache every packet, every session lookup, and every
packet-processing-progress structure.  Massively parallelize, spinlock
on on-chip SRAM, tune for the cold-cache case.  If you can't afford to
do that -- and if you haven't designed your own chip, with separate
cache windows for each of these use cases, you can't, because they're
all retrograde loads for an LRU cache -- then a hash is not the right
answer.  The interaction between resizing and RCU is just the canary
in the coal mine.

Cheers,
- Michael
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