Date: Wed, 15 Sep 2010 15:19:50 +0000 (UTC) From: "Bjoern A. Zeeb" <bzeeb-lists@lists.zabbadoz.net> To: Andre Oppermann <oppermann@networx.ch> Cc: freebsd-net@freebsd.org, freebsd-current@freebsd.org Subject: Re: TCP loopback socket fusing Message-ID: <20100915151632.E31898@maildrop.int.zabbadoz.net> In-Reply-To: <4C8E0C1E.2020707@networx.ch> References: <4C8E0C1E.2020707@networx.ch>
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On Mon, 13 Sep 2010, Andre Oppermann wrote: Hey, > When a TCP connection via loopback back to localhost is made the whole > send, segmentation and receive path (with larger packets though) is still > executed. This has some considerable overhead. > > To short-circuit the send and receive sockets on localhost TCP connections > I've made a proof-of-concept patch that directly places the data in the > other side's socket buffer without doing any packetization and other protocol > overhead (like UNIX domain sockets). The connections setup (SYN, SYN-ACK, > ACK) and shutdown are still handled by normal TCP segments via loopback so > that firewalling stills works. The actual payload data during the session > won't be seen and the sequence numbers don't move other than for SYN and FIN. > The sequence are remain valid though. Obviously tcpdump won't see any data > transfers either if the connection has fused sockets. > > Preliminary testing (with WITNESS and INVARIANTS enabled) has shown stable > operation and a rough doubling of the throughput on loopback connections. > I've tested most socket teardown cases and it behaves fine. I'm not entirely > sure I've got all possible path's but the way it is integrated should > properly > defuse the sockets in all situations. Three comments in reverse order: 1 If S/S+A/A and shutdown aren't shortcut, can you always rely on proper payload order, especially in the shutdown case? 2 Given my experience with epairs, which are basically a loop with two interfaces and even interface queues, any significant delay you are seeing is _not_ due to longer code paths through the stack but simply because of the netisr. 3 If properly doing this for TCP, we should probably also do it for other protocols. /bz -- Bjoern A. Zeeb Welcome a new stage of life.
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