jamal wrote:
On Thu, 27 Feb 2003, Chris Friesen wrote:
It is fairly common to want to distribute information between a single sender and multiple receivers on a single box.
Multicast IP sockets are one possibility, but then you have additional overhead in the IP stack.
From lmbench local communication tests:I think this is a _very weak_ reason. Without addressing any of your other arguements, can you describe what such painful overhead you are talking about? Did you do any measurements and under what circumstances are unix sockets vs say localhost bound udp sockets are different? I am not looking for hand waving reason of "but theres an IP stack".
This is a multiproc 1GHz G4
Host OS 2p/0K Pipe AF UDP RPC/ TCP RPC/ TCP
ctxsw UNIX UDP TCP conn
--------- ------------- ----- ----- ---- ----- ----- ----- ----- ----
pcary0z0. Linux 2.4.18- 0.600 3.756 6.58 10.2 26.4 13.8 36.9 599K
pcary0z0. Linux 2.4.18- 0.590 3.766 6.43 10.1 26.7 13.9 37.2 59.1
This is a 400MHz uniproc G4
Host OS 2p/0K Pipe AF UDP RPC/ TCP RPC/ TCP
ctxsw UNIX UDP TCP conn
--------- ------------- ----- ----- ---- ----- ----- ----- ----- ----
zcarm0pd. Linux 2.2.17- 1.710 9.888 21.3 26.4 59.4 43.0 105.4 146.
zcarm0pd. Linux 2.2.17- 1.740 9.866 22.2 26.3 60.4 43.1 106.7 147.
This is a 1.8GHz P4
Host OS 2p/0K Pipe AF UDP RPC/ TCP RPC/ TCP
ctxsw UNIX UDP TCP conn
--------- ------------- ----- ----- ---- ----- ----- ----- ----- ----
pcard0ks. Linux 2.4.18- 1.740 10.4 15.9 20.1 33.1 23.5 44.3 72.7
pcard0ks. Linux 2.4.18- 10.3 16.1 19.8 36.3 22.8 43.6 74.1
pcard0ks. Linux 2.4.18- 1.560 10.6 16.0 23.4 38.1 36.1 44.6 77.4
From these numbers, UDP has 18%-44% higher latency than AF_UNIX, with the difference going up as the machine speed goes up.
Aside from that, IP multicast doesn't seem to work properly. I enabled multicast on lo and disabled it on eth0, and a ping to 224.0.0.1 still got responses from all the multicast-capable hosts on the network. From userspace, multicast unix would be *simple* to use, as in totally transparent.
The other reason why I would like to see this happen is that it just makes *sense*, at least to me. We've got multicast IP, so multicast unix for local machine access is a logical extension in my books.
Do we agree at least that some form of multicast is the logical solution to the case of one sender/many listeners?
Thanks for your thoughts,
Chris
--
Chris Friesen | MailStop: 043/33/F10
Nortel Networks | work: (613) 765-0557
3500 Carling Avenue | fax: (613) 765-2986
Nepean, ON K2H 8E9 Canada | email: cfriesen@nortelnetworks.com
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