Re: anyone ever done multicast AF_UNIX sockets?

From: jamal (
Date: Sun Mar 02 2003 - 09:11:39 EST

On Sun, 2 Mar 2003, Chris Friesen wrote:

> jamal wrote:
> > Did you also measure throughput?
> No. lmbench doesn't appear to test UDP socket local throughput.

I think you need to collect all data if you are trying to show

> > You are overlooking the flexibility that already exists in IP based
> > transports as an advantage; the fact that you can make them
> > distributed instead of localized with a simple addressing change
> > is a very powerful abstraction.
> True. On the other hand, the same could be said about unicast IP
> sockets vs unix sockets. Unix sockets exist for a reason, and I'm
> simply proposing to extend them.

You are treading into areas where unix sockets make less sense compared to
sockets. Good design rules (should actually read "lazy design
rules") ometimes you gotta move to a round peg instead of trying to make
the square one round.

> > You could implement the abstraction in user space as a library today by
> > having some server that muxes to several registered clients.
> This is what we have now, though with a suboptimal solution (we
> inherited it from another group). The disadvantage with this is that it
> adds a send/schedule/receive iteration. If you have a small number of
> listeners this can have a large effect percentage-wise on your messaging
> cost. The kernel approach also cuts the number of syscalls required by
> a factor of two compared to the server-based approach.

Ok, so its only a problem when you have a few listeners i.e user space
scheme scales just fine as you keep adding listeners.
In your tests what was the break-even point?

> > So whats the addressing scheme for multicast unix? Would it be a
> > reserved path?
> Actually I was thinking it could be arbitrary, with a flag in the unix
> part of struct sock saying that it was actually a multicast address.
> The api would be something like the IP multicast one, where you get and
> bind a normal socket and then use setsockopt to attach yourself to one
> or more of multicast addresses. A given address could be multicast or
> not, but they would reside in the same namespace and would collide as
> currently happens. The only way to create a multicast address would be
> the setsockopt call--if the address doesn't already exist a socket would
> be created by the kernel and bound to the desired address.

Addressing has to be backwared compatible i.e not affecting any other

> To see if its feasable I've actually coded up a proof-of-concept that
> seems to do fairly well. I tested it with a process sending an 8-byte
> packet containing a timestamp to three listeners, who checked the time
> on receipt and printed out the difference.
> For comparison I have two different userspace implementations, one with
> a server process (very simple for test purposes) and the other using an
> mmap'd file to store which process is listening to what messages.
> The timings (in usec) for the delays to each of the listeners were as
> follows on my duron 750:
> userspace server: 104 133 153
> userspace no server: 72 111 138
> kernelspace: 60 91 113
> As you can see, the kernelspace code is the fastest and since its in the
> kernel it can be written to avoid being scheduled out while holding
> locks which is hard to avoid with the no-server userspace option.

Actually, the difference between user space server and kernel doesnt
appear that big. What you need to do is collect more data.
repeat with incrementing number of listeners.

> If this sounds at all interesting I would be glad to post a patch so you
> could shoot holes in it, otherwise I'll continue working on it privately.

no rush, lets see your test data first and then you gotta do a better
sales job on the cost/benefit/flexibilty ratios.

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This archive was generated by hypermail 2b29 : Fri Mar 07 2003 - 22:00:00 EST