netdev - Re: [RFC] [PATCH] udp: optimize lookup of UDP sockets to by including destination address in the hash key

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Date:	Thu, 5 Nov 2009 18:25:45 +0200
From:	Octavian Purdila <opurdila@...acom.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Lucian Adrian Grijincu <lgrijincu@...acom.com>,
	netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] udp: optimize lookup of UDP sockets to by including destination address in the hash key

On Thursday 05 November 2009 01:32:18 you wrote:

> >
> > Very true, the benchmark itself shows a significant overhead increase on
> > the TX side and indeed this case is not very common. But for us its an
> > important usecase.
> >
> > Maybe there is a more clever way of fixing this specific use-case without
> > hurting the common case?
> 
> Clever way ? Well, we will see :)
> 
> I now understand previous Lucian patch (best match) :)
> 
> Could you please describe your usecase ? I guess something is possible,
> not necessarly hurting performance of regular usecases :)
> 

IIRC, we first saw this issue in VoIP tests with up to 16000 sockets bound on a 
certain port and IP addresses (each IP address is assigned to a particular 
interface). We need this setup in order to emulate lots of VoIP users each 
with a different IP address and possible a different L2 encapsulation.

Now, as a general note I should say that our usecases can seem absurd if you 
take them out of the network testing field :) but my _personal_ opinion is that 
a better integration between our code base and upstream code may benefit both 
upstream and us:

- for us it gives the ability to stay close to upstream and get all of the new 
shiny features without painful upgrades

- for upstream, even if most systems don't run into these scalability issues 
now, I see that some people are moving in that direction (see the recent PPP 
problems); also, stressing Linux in that regard can only make the code better 
- as long as the approach taken is clean and sound

- we (or our customers) use a plethora of networking devices for testing so 
exposing Linux early to those devices can only help catching issues earlier

In short: expect more absurd patches from us :) 

> I have struct reorderings in progress to reduce number of cache lines read
> per socket from two to one. So this would reduce by 50% time to find
> a particular socket in the chain.
> 
> But if you *really* want/need 512 sockets bound to _same_ port, we probably
>  can use secondary hash tables (or rbtree), as soon as we stack more than
>  XX sockets on a particular slot.
> 
> At lookup, we check if extended hash table exists before doing
> normal rcu lookup.
> 
> Probably can be done under 300 lines of code.
> On normal machines, these extra tables/trees would not be used/allocated
> 

Yep, that should work. Will respin the patch based on this idea and see what 
we get, but it will take a while.

Thanks,
tavi

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