netdev - Re: [PATCH net-next 2/2] udp: Reset flow table for flows over unconnected sockets

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Date:	Mon, 27 Oct 2014 11:43:57 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 2/2] udp: Reset flow table for flows over
 unconnected sockets

On Mon, 2014-10-27 at 11:01 -0700, Tom Herbert wrote:
> When receiving a packet on an unconnected UDP socket clear the
> flow table for the corresponding hash. This is needed so flows over
> unconnected UDP sockets will use RPS instead of using what is
> present in the flow table. In particular, this avoids having flows
> over unconnected sockets be perpetually steered by unrelated
> entries in the flow table (idle TCP connections for instance).
> 
> Tested:
> 
> First filled up the RPS flow tables by creating a bunch of TCP
> connections and letting them turn idle. Next, run netperf UDP_RR
> with 200 flows.
> 
> Before fix:
>   Client (connected UDP)
>     81.15% CPU uilization
>   Server (unneconnedted UDP)
>     83.63% CPU uilization
>   118/167/249 90/95/99% latencies
>   1.59215e+06 tps
> 
> After fix:
>   Client (connected UDP)
>     81.13% CPU uilization
>   Server (unneconnedted UDP)
>     80.68% CPU uilization
>   116/167/248 90/95/99% latencies
>   1.61048e+06 tps
> 
> Signed-off-by: Tom Herbert <therbert@...gle.com>
> ---
>  net/ipv4/udp.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 9a0d346..e58d841 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1451,6 +1451,11 @@ static int __udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
>  	if (inet_sk(sk)->inet_daddr) {
>  		sock_rps_save_rxhash(sk, skb);
>  		sk_mark_napi_id(sk, skb);
> +	} else {
> +		/* For an unconnected socket reset flow hash so that related
> +		 * flow will use RPS.
> +		 */
> +		sock_rps_reset_flow_hash(skb->hash);
>  	}

I believe I already said this patch was wrong Tom.

We need something else for UDP packets.

Its not because RFS is wrong for UDP packets that we want to make it
worse for TCP traffic.

We do now want UDP packets to gradually make flow table empty.




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