Hello,
I would like to realize something similar to a real radio broadcast:
- a publisher distributes packets of information
- subscribers receive and process information
but in my case timing is very important; to keep the radio analogy:
- all subscribers receive the same information pretty much at the same time
- if a subscriber stops listening for a few seconds, it loses some words but
does not lose synchronization with others (when he starts listening again, he's
where every one else is in the story)
I thought that setting a very low HWM (1) in a publisher-subscriber socket
would pretty much solve my problems, but instead I find that HWM is somewhat
ignored.
A very very ingenious and terribly coded example would have, on the server
side, a radio broadcasting a sequential number and a timestamp, in a somewhat
verbose way to increase the package size to some credible value:
import zmq
import time
context = zmq.Context()
socket = context.socket(zmq.PUB)
socket.setsockopt(zmq.SNDHWM, 1)
socket.setsockopt(zmq.RCVHWM, 1)
socket.bind("tcp://*:9999")
counter = 0
while True:
msg = "%d-%s " % (counter,time.time())*100
socket.send(msg)
print 'sent ', msg[0:20].split(' ')[0]
counter += 1
time.sleep(0.005)
And the listener, instead, hears a message, and wonders how long it took to get
the message:
import zmq
import time
# Socket to talk to server
context = zmq.Context()
socket = context.socket(zmq.SUB)
socket.setsockopt(zmq.SNDHWM, 1)
socket.setsockopt(zmq.RCVHWM, 1)
socket.setsockopt(zmq.DELAY_ATTACH_ON_CONNECT, 1)
socket.setsockopt(zmq.SUBSCRIBE, '')
socket.connect ("tcp://localhost:9999")
while True:
msg = socket.recv()
t = time.time()
count, mt = msg[0:25].split(' ')[0].split('-')
print 'got: ', str(msg[0:25].split(' ')[0]), ' at: ', t, '
len: ', len(msg) , ' delay: ', t - float(mt)
time.sleep(0.5)
What I would expect in this case, is indeed to see a lot of packet loss on the
receiver side, but very small delay.
Instead, what happens is that somehow there is no packet loss and the delay
increases to very unreasonable values, an output example from the client is:
[...]
got: 325-1384093263.38 at: 1384093264.38 len: 1800 delay:
0.997805833817
got: 326-1384093263.39 at: 1384093265.38 len: 1800 delay: 1.98881793022
got: 327-1384093263.39 at: 1384093266.38 len: 1800 delay: 2.98981881142
got: 328-1384093263.4 at: 1384093267.38 len: 1700 delay: 3.98081588745
got: 329-1384093263.4 at: 1384093268.38 len: 1700 delay: 4.98182487488
got: 330-1384093263.41 at: 1384093269.38 len: 1800 delay: 5.97282195091
got: 331-1384093263.41 at: 1384093270.38 len: 1800 delay: 6.97381591797
got: 332-1384093263.42 at: 1384093271.38 len: 1800 delay: 7.96481204033
got: 333-1384093263.42 at: 1384093272.39 len: 1800 delay: 8.96581697464
got: 334-1384093263.43 at: 1384093273.39 len: 1800 delay: 9.95681405067
got: 335-1384093263.43 at: 1384093274.39 len: 1800 delay: 10.9578130245
got: 336-1384093263.44 at: 1384093275.39 len: 1800 delay: 11.9488120079
got: 337-1384093263.44 at: 1384093276.39 len: 1800 delay: 12.9498069286
got: 338-1384093263.45 at: 1384093277.39 len: 1800 delay: 13.9408349991
got: 339-1384093263.45 at: 1384093278.39 len: 1800 delay: 14.9418139458
[...]
meanwhile the server already printed:
[...]
sent 3263-1384093278.39
sent 3264-1384093278.4
sent 3265-1384093278.4
sent 3266-1384093278.41
sent 3267-1384093278.41
sent 3268-1384093278.42
sent 3269-1384093278.42
[...]
Changing the network buffer size even to unreasonably small settings, for
example:
socket.setsockopt(zmq.SNDBUF, 256)
socket.setsockopt(zmq.RCVBUF, 256)
on both ends, does have very little effect unless the packets you send are much
bigger then the buffer (try changing the multiplier to 9999 or more): in that
case the receiver barely receives any packages, but still with somewhat big
delay.
This said, anybody would like to help me to understand what's going on here?
Maybe there is a way to eat all broadcasted packets in one go and keep only the
last one?
Is this configuration of any practical interest to anyone?
I'm on debian sid, python 2.7.5 and zmq version 3.2.4.
Thank you very much!
Cheers,
Samuele
--
|--
| Samuele Carli
|--
| Contacts:
|
| Home page : www.csspace.net
| E-mail : carlisamuele _at_ csspace.net
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