netdev - Re: [PATCH net-next] net: rfs: add hash collision detection

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Date:	Sun, 08 Feb 2015 16:54:27 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, therbert@...gle.com, ycai@...gle.com,
	willemb@...gle.com
Subject: Re: [PATCH net-next] net: rfs: add hash collision detection

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 06 Feb 2015 12:59:01 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Receive Flow Steering is a nice solution but suffers from
> hash collisions when a mix of connected and unconnected traffic
> is received on the host, when flow hash table is populated.
> 
> Also, clearing flow in inet_release() makes RFS not very good
> for short lived flows, as many packets can follow close().
> (FIN , ACK packets, ...)
> 
> This patch extends the information stored into global hash table
> to not only include cpu number, but upper part of the hash value.
> 
> I use a 32bit value, and dynamically split it in two parts.
> 
> For host with less than 64 possible cpus, this gives 6 bits for the
> cpu number, and 26 (32-6) bits for the upper part of the hash.
> 
> Since hash bucket selection use low order bits of the hash, we have
> a full hash match, if /proc/sys/net/core/rps_sock_flow_entries is big
> enough.
> 
> If the hash found in flow table does not match, we fallback to RPS (if
> it is enabled for the rxqueue).
> 
> This means that a packet for an non connected flow can avoid the
> IPI through a unrelated/victim CPU.
> 
> This also means we no longer have to clear the table at socket
> close time, and this helps short lived flows performance.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks Eric.
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