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Re: anyone ever done multicast AF_UNIX sockets?

To: Terje Eggestad <terje.eggestad@xxxxxxxxx>
Subject: Re: anyone ever done multicast AF_UNIX sockets?
From: Chris Friesen <cfriesen@xxxxxxxxxxxxxxxxxx>
Date: Mon, 03 Mar 2003 17:29:14 -0500
Cc: linux-kernel <linux-kernel@xxxxxxxxxxxxxxx>, netdev@xxxxxxxxxxx, linux-net@xxxxxxxxxxxxxxx, davem@xxxxxxxxxx
References: <3E5E7081.6020704@nortelnetworks.com> <1046695876.7731.78.camel@pc-16.office.scali.no> <3E638C51.2000904@nortelnetworks.com> <1046720360.28127.209.camel@eggis1>
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Terje Eggestad wrote:
On Mon, 2003-03-03 at 18:09, Chris Friesen wrote:
Terje Eggestad wrote:
> On a single box you would use a shared memory segment to do this. It has
> the following advantages:
> - no syscalls at all
Unless you poll for messages on the receiving side, how do you trigger the receiver to look for a message? Shared memory doesn't have file descriptors.
OK, you want multicast to send the *same* info to all peers. The only of
two sane reason to do that is to update the peers with some info they
need to do real work. So when there is reel work to be done, the info is
available in the shm.

Okay, but how do they know there is work to be done? They're waiting in select() monitoring sockets, fds, being hit with signals, etc. How do you tell them to check their messages? You have to hit them over the head with a signal or something and tell them to check the shared memory messages.
If you *had* multicast, you don't know *when* a peer proccessed it. What if the peer is suspended ??? you don't get an error on the send,
and you apparently never get an answer, then what? The peer may also
gone haywire on a while(1);

Exactly. So if the message got delivered you have no way of knowing for sure that it was processed and you have application-level timers and stuff. But if the message wasn't delivered to anyone and you know it should have been, then you don't have to wait for the timer to expire to know that they didn't get it.


How do they know the information has changed? Suppose one process detects that the ethernet link has dropped. How does it alert other processes which need to do something?
Again, if you want someone to do something, they must ack the request
before you can safely assume that they are going to do something.

Certainly. My point was that if you're trying to handle all events in a single thread, you need some way to tell the message recipient that it needs to check the shared memory buffer. Otherwise you need multiple threads like you mentioned in your project description.



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@xxxxxxxxxxxxxxxxxx


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