netdev - Re: Extensible hashing and RCU

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Message-ID: <20070222112451.47490630@freekitty>
Date:	Thu, 22 Feb 2007 11:24:51 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	David Miller <davem@...emloft.net>
Cc:	medwards.linux@...il.com, johnpol@....mipt.ru, dada1@...mosbay.com,
	akepner@....com, linux@...izon.com, netdev@...r.kernel.org,
	bcrl@...ck.org
Subject: Re: Extensible hashing and RCU

On Thu, 22 Feb 2007 03:07:55 -0800 (PST)
David Miller <davem@...emloft.net> wrote:

> From: "Michael K. Edwards" <medwards.linux@...il.com>
> Date: Thu, 22 Feb 2007 03:00:32 -0800
> 
> > Besides, RCUing a hash of any sort sounds like a nightmare, whereas
> > something like a splay tree is reputed to be very straightforward to
> > make "persistent" in the functional-programming sense.  I haven't
> > coded that one myself but surely it's in the current edition of Knuth.
> 
> Knuth tends to be thorough, yet archaic. :-)
> 
> > You are much better off obtaining all per-flow state with a
> > single data structure lookup.  It would be kind of nice if state for
> > other layers like IPSec and NAT could live there too.
> 
> This is the eventual idea.
> 
> > By the way, are these flows unidirectional or bidirectional?
> 
> The entries created by Robert's TRASH are unidirectional, they have to
> be because the path from remote to localhost is different than the
> path from localhost to remote :-)
> 
> But we don't need to do a socket demux on packet output, we just
> need a plain route (+ firewall, IPSEC, etc.) destination cache
> entry for that.
> -


There are a number of other lock-less algorithms rather than simple RCU
that might be worth exploring. A number of them rely on a working implementation
of compare-exchange; that can be a problem since some architectures don't have
a fast way of doing that.

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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