netdev - Re: neighbour table RCU

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Date:	Tue, 1 Sep 2009 08:59:21 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: neighbour table RCU

On Tue, 01 Sep 2009 08:50:17 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Stephen Hemminger a écrit :
> > Looking at the neighbour table, it should be possible to get
> > rid of the two reader/writer locks.  The hash table lock is pretty
> > amenable to RCU, but the dynamic resizing makes it non-trivial.
> > Thinking of using a combination of RCU and sequence counts so that the
> > reader would just rescan if resize was in progress.
> 
> I am not sure neigh_tbl_lock rwlock should be changed, I did not
> see any contention on it.
> 
> > 
> > The reader/writer lock on the neighbour entry is more of a problem.
> > Probably would be simpler/faster to change it into a spinlock and
> > be done with it.
> > 
> > The reader/writer lock is also used for the proxy list hash table,
> > but that can just be a simple spinlock.
> > 
> 
> This is probably is the only thing we want to do at this moment,
> halving atomic ops on neigh_resolve_output()
> 
> But why neigh_resolve_output() was called so much in the bench
> is the question...
> 

Every packet has to have an ARP resolution.

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