[rabbitmq-discuss] RabbitMQ and Stomp topic behavior

Sun Sep 18 13:56:52 BST 2011

Massimo,
After quite a  time of trials and tribulations I have found a bug 
in the RabbitMQ STOMP adapter which appears to cause 
crashes under high message load.
    The bug is related to the (rather primitive) throttling which 
kicks in when reaching the memory limit.

After fixing this, I have successfully run your testcase, and 
RabbitMQ STOMP does manage to survive during the throwing 
away of large numbers of unsubscribed topic SENDs. I had to 
make one alteration to the configuration in order to get the test 
to stay up. I set the vm_high_watermark to 0.2 (instead of the 
default of 0.4).

This setting determines the proportion of available memory 
above which the 'memory-alarm' is triggered. The reader 
process in RabbitMQ-STOMP will then block processing 
further bytes from clients, until the memory alarm is switched 
off.  Because message processing is proceeding at a very 
high rate (that is why memory is being used up), the 
input-queues for Erlang processes are likely to be large, 
and so even blocking input will not result in a lowering of 
memory consumption -- memory requirements increase in 
the pipeline flow -- and so the actual memory consumed is 
likely to reach 0.6/0.7 of available memory (found by observation).  
This is why setting it to 0.4 (the default) doesn't work.  Memory 
for Erlang can then be completely exhausted, resulting in 
crashes (which normally result in long pauses and large 
amounts of dump-data written to disk). Any recovery after 
that is likely not to succeed for very long, since the pause 
allows time for more requests to be put in. (The benchmark 
test runs with non-blocking io; I'm not sure what that means, 
but if it means that requests will be put in even if the socket 
blocks then this is going to increase the load during the 
pauses.)

As I say, setting it to 0.2 worked, and I suspect that 0.3 would 
still work.

However,...

The test doesn't really tell us much.  This is a poor benchmark, 
because it consists of a single producer sending a 20-byte 
message to a topic to which no-one is currently subscribed.  
This is therefore a measure of how fast a broker can throw 
away messages.  RabbitMQ does reasonably well here, but 
cannot determine that there are no subscribers until it reaches 
the actual rabbit exchange, so it goes through AMQP 
processing.  In general, especially in the presence of potential 
durable topic subscriptions, it is hard to see how to fix this -- 
though if it were a common scenario perhaps some sort of 
status of the broker subscription space could be cached near 
to the client, allowing us to throw away messages close to 
the client: but then we need to update the cache when 
(remote) subscribers come on line, or risk throwing away 
messages in error.  This is very complex, and only for a gain 
in a low probability use-case.

I notice that the benchmark test runs for a very long time 
(hours on my little 2Gb 2Core Linux VM) and produces a noisy 
graph as result (there are lots of zeros in it because RabbitMQ 
blocks reads quite often). The graph doesn't help us to guess 
at an average rate.  I attach the benchmark json output, so 
you can view it with generic_report, or something.

The bug fix is rabbit bugzilla 24426, which after QA will 
probably go into the next release.

Thanks for spotting this,

Steve Powell
steve at rabbitmq.com
[wrk: +44-2380-111-528] [mob: +44-7815-838-558]

Previously:
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