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SubjectRe: anyone ever done multicast AF_UNIX sockets?
jamal wrote:
> 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
> improvements.

I'll look at how they were measuring unix socket throughput and try
implementing something similar for UDP. It's not clear to me how to
really measure throughput in a multicast environment though since it
depends very much on your application messaging patterns.

> 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?

See below for more detailed test results.

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

Of course. The way I've designed it is that you get and bind() a socket
as normal, and then use setsockopt() to register interest in a multicast
address (same as IP multicast). If the address already exists but is
not a multicast address, then you get an error. If a socket tries to
bind() or connect() to an existing multicast address, you get an error.
The different types of addresses exist in the same address space, but
the only way to register interest in multicast addresses is through

>>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

> 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.

What would you consider a "big" difference? Here the userspace server
is 35% slower than the kernelspace version.

You wanted more data, so here's results comparing the no-server
userspace method vs the kernel method. The server-based one would be
slightly more expensive than the no-server version. The results below
are the smallest and largest latencies (in usecs) for the message to
reach the listeners in userspace. I've used three different sizes, the
two extremes and a roughly average sized message in my particular domain.

# listeners userspace kernelspace
10 73,335 103,252
20 72,610 106,429
50 74,1482 205,1301
100 76,3000 362,3425
200 737,9917

# listeners userspace kernelspace
10 70,346 81,265
20 74,639 122,468
50 75,1557 230,1421
100 80,3107 408,3743

40036-byte message
# listeners userspace kernelspace
10 302,4181 322,1692
20 303,7491 347,3450
50 306,10451 483,8394
100 309,23107 697,17061
200 313,45528 997,39810

As one would expect, the initial latencies are somewhat higher for the
kernel space solution since all the skb header duplication is done
before anyone is woken up. One thing that I did not expect was the
increased max latency in the kernel space soltion when the number of
listeners grew large. On reflection, however, I suspect that this is
due to scheduler load since all of the listening processes have become
runnable while in the userspace version they become runnable one at a
time. It would be interesting to run this on 2.5 with the O(1)
scheduler and see if it makes a difference.

With larger message sizes, the cost of the additional copies in the
userspace solution start to outweigh the overhead of the additional
runnable processes and the kernel space solution stays faster in all
runs tested.


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:

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