netdev - Re: neighbour table RCU

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Date:	Tue, 1 Sep 2009 19:56:45 +0300
From:	Octavian Purdila <opurdila@...acom.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	Lucian Adrian Grijincu <lgrijincu@...acom.com>,
	netdev@...r.kernel.org
Subject: Re: neighbour table RCU

On Tuesday 01 September 2009 19:14:37 Eric Dumazet wrote:
> Octavian Purdila a écrit :
> > On Tuesday 01 September 2009 09:50:17 Eric Dumazet 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.
> >
> > Speaking about neighbour optimizations, here is a RFC patch which makes
> > the tables double linked, for constant time deletion. It has given us a
> > significant performance improvement - in less then usual setups though,
> > with lots of neighbours.
>
> How many "struct neigh_parms" do you have in your setups, and
> what is the frequency of neigh_parms_release() calls ???
>

Each L3 interface part (at least for IPv4/IPv6 and I see DecNet as well) has a 
neigh_parms associated. And we use up to 128K interfaces in certain tests 
scenarios ;)

In our case the interfaces are created/destroyed dynamically, so when using a 
large number of interfaces the test cleanup takes forever without this (and 
other) patch(es).

Thanks,
tavi
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