[rabbitmq-discuss] Queue depth and no. of consumers.

michael neidhardt meem.ok at gmail.com
Tue Nov 1 07:43:05 GMT 2011

OK - will upgrade and see if that removes the RAM problem.

Thanks for your help,

On 31 Okt., 17:27, Simon MacMullen <si... at rabbitmq.com> wrote:
> On 31/10/11 14:31, michaelneidhardtwrote:
> > With no QoS set, I assume each consumer fetches as many messages as
> > possible (whatever that means),
> > and 'queues' them itself.
> Yes, although the "queueing" doesn't mean there's an explicit queue in
> the client. But messages can back up in networking buffers etc.
> > I will try to set QoS/prefetch_count to 1,
> > though I seem to remember
> > having read that this can cause trouble. I would assume that it would
> > be no problem, have you got any thoughts on whether it's a good idea?
> I don't see why it should cause trouble. Setting prefetch to 1 does mean
> that throughput will drop since the broker cannot deliver another
> message until the previous one has been acknowledged, so if throughput
> is a concern then you might set the prefetch to some larger number for
> less fairness but more performance.
> In general setting the prefetch count is a good idea if you're concerned
> with fair distribution of messages.
> > As I wrote earlier, we have a number (several thousand) of files to
> > process, each of which may contain up to several hundred thousand
> > records (around 2KB each).
> > In an earlier test, I let the processes handling files push the ID of
> > each record to a queue.
> > (Simply add a publish to the above code after the ack). The consumer
> > for this queue (which uses autoack)
> (Of course, setting prefetch doesn't do anything in noack mode since the
> server has no way to know how many messages have made it all the way to
> the client.)
> > would do a lookup in a Postgresql
> > DB, and nothing else.
> > After about 50 million records, the vm_memory_high_watermark would be
> > set, and shortly after
> > that, I got<"timeout waiting for channel.flow_ok{active=false}",none}
> >> . Eventually the whole system froze.
> > I guess this timeout is caused by my client not reacting to the flow
> > control from the RabbitMQ server. Is that correct? Unfortunately, the
> > client I use does not have methods for that. Should I expect to handle
> > this in normal operation, or could it be handled by a client for me?
> I think you said you were using 1.8.1. Much has changed since then:
> * We no longer use channel.flow to throttle producers since as you're
> seeing many clients did not implement it correctly / at all. We now use
> TCP backpressure instead.
> * Prior to 2.0 all messages in all queues had to fit in memory. Messages
> are now paged to disk when memory is low.
> So I would strongly advise you to upgrade to 2.6.1. It's available in
> our apt repository:
> http://www.rabbitmq.com/debian.html
> > Oh, and the big question: Is it out of the question to handle approx.
> > 300 mill. messages (where payload is essentially a bigint) over a few
> > days?
> That should be no big deal. My workstation can churn through that in a
> few hours.
> Cheers, Simon
> --
> Simon MacMullen
> RabbitMQ, VMware
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