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From drvillo <f.viv...@gmail.com>
Subject Flow control on a recursive system, guidelines?
Date Fri, 29 Dec 2006 11:45:37 GMT

Hi all

I have been trying to get a stable configuration for over a month now, but
I keep encountering a variety of issues[1] that reduces everything to an
unutilizable 
system.
I have now built a simpler system, which perhaps exposes the same behavior,
it is just a pipeline of two queues

seeds--->q1--->L1--->q2--->L2

what happens is that when L1 receives a message it sends a number of
messages to q2.
I run tests changing the number of messages to be sent to q1 and the number
of
messages to be forwarded by L1 to q1 (toSend and toForward respectively)

A first scenario representing a real use case is having toSend small (1-10)
and toForward
big(10k-100k)

A first observation is that if using kaha persistence everything eventually
stops, when the broker
starts to be too full[2] of messages, having the attached dump. Thus I'm
using the default
jdbc persistence.

A general behavior that I've seen is that if toSend is big (10k) then no
flow control 
seems to take place, as L2 eventually stops receiving messages when the
broker
runs out of memory (when not using an usagemanageer), or the heap space is
exahusted
(when setting it to something like 1Gb).
In this case I'd expect that sends to L1 are slowed down, to allow L2 to
consume the (more) messages
that are arriving. but this doesn't happen, seeds are kept being sent at the
same rate.

On the other hand having toSend<<toForward makes all the messages to be
delivered,
but after some time everything slows awfully down (1msg/second maybe...),
mainly because
the jvm is garbage collecting all the time (the heap space is being all used
up).


Basically I need some way to slow down the first producer so that the whole
system is not flooded
with messages that then I can barely consume.
My question then is, how should I configure the destination and broker
memory limits, and eventually
the prefetch values so that I don't either run out of memory or end up with
a frozen system?

Setting higher memory limits causes the JVM to need a bigger heap space
(which makes it less stable
in uncontrolled environments) but keeping them low seems that prevents the
broker 
to send more messages at some point (again [1]). On the other side, being
everything in the 
same VM, I don't know if it's better to set prefetch limits higher or lower,
as pending messages
have to be stored somewhere, either on the broker or on the consumer...
Reading about slow consumers[3] doesn't point me to any option, I can't
discard messaging.
So I end up considering implementing a sort of timing between sends, or
waiting for
message spooling, which I read should come with 4.2...


Sorry for the long message, but it's hard to express all of the above, there
are probably too
many options to be considered. 
Any help, guideline or hint would be most apreciated, it seems that this
project is not
going to be released ever:(
For the interested is http://www.ripe.net/info/stats/hostcount/hostcount++/


Thanks everybody,
Francesco

[1]: http://www.nabble.com/Consumer-Deadlock-tf2014492.html#a5536571
[2]: I haven't got any measure for too full, but I'd say something like 800k
messages,
with an heap space of 128Mb
[3]:  http://activemq.org/site/slow-consumer-handling.html
http://www.nabble.com/file/5167/kahadump.log kahadump.log 
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