netdev - Re: RFC udp: improve __udp4_lib

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Message-ID: <1334239215.5300.6475.camel@edumazet-glaptop>
Date:	Thu, 12 Apr 2012 16:00:15 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexandru Copot <alex.mihai.c@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: RFC udp: improve __udp4_lib_lookup performance

On Thu, 2012-04-12 at 16:35 +0300, Alexandru Copot wrote:
> UDP uses 2 hashtables for fast socket lookup. First hash uses port as
> a lookup key and the second one uses (port, addr).
> 
> When an UDP packet is received, the destination socket must be found to
> deliver it. If there are many UDP sockets bound to INADDR_ANY, 2 hash
> searches are made in the second hash: first one looks for the pair
> (dest address, dest port) but doesn't find the socket; the second search
> finds the socket by hashing (INADDR_ANY, dest port).
> 
> Those 2 searches can be avoided and a lot of time saved if instead we
> searched directly in the first hash.
> 
> We could count the number of INADDR_ANY bound UDP sockets and
> make only one search when that value is above a certain threshold. However,
> if there are also sockets bound on a specific address, the second hash
> won't be used and that might hurt performance for this case.
> 
> What is your opinion on this ? Would the performance gained by
> counting INADDR_ANY bound sockets outweigh the loss in performance
> for the case of mixed INADDR_ANY/specific address bound sockets ?

I have no idea of your workload, but existing code is already very
optimized.

udp4_lib_lookup2() variants are only called when (hslot->count > 10)

If your workload have one 60000 UDP sockets bound on INADDR_ANY, I
suggest you check udp hash size and eventually increase it to the max ?

dmesg | grep "UDP hash"

Boot command : uhash_entries=65536




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