On 03/14/2012 02:20 PM, Jeff Armstrong wrote:
Answers are inline.
________________________________________
From: Gordon Sim [[email protected]]
Sent: Wednesday, March 14, 2012 6:39 AM
To: [email protected]
Subject: Re: Major slowdown on Qpid broker
On 03/13/2012 10:35 PM, Jeff Armstrong wrote:
[...]
If an active blade goes down, the standby blade becomes active, the receiver
clients there will now connect to the brokers and start dequeuing,
How do the published messages get to the other broker?
Jeff: In this case, the unacquired messages on the broker of the blade that
goes down will be lost, which is expected at this point.
So which messages do you expect to be redelivered?
while the other blade will eventually reboot into standby mode.
The two receiver clients each subscribe to their own single queue, which are
attached to the same binding on the same exchange. The clients' normal
behaviour is to dequeue messages for 15 minutes, finish processing them, then
send an accept() on the subscription of all the processed id's. The idea is
that if the active blade goes down, all of the messages that were not
accept()ed will be lost, so the clients on the standby blade will then connect
and should get these messages redelivered. This seems to have worked in the
past.
How do those messages that were not accepted get to the standby broker?
The following events occurred (note that only blade 1 is actually enqueuing to
its broker, blade 2 has no enqueuing going on, this is on purpose):
- blade 1 (active) and blade 2 (standby)
- blade 1 reboots, so blade 2 becomes active then blade 1 comes up into standby
- blade 2 then reboots, so blade 1 becomes active then blade 2 goes into standby
We then made the following observations:
- When blade 2 reboots, and blade 1 becomes active, the receiver clients never
output any of the expected redelivered messages. We think that the redelivery
never took place.
- When inspecting the 'unacked' queue in SemanticState (and also the queue in
RingQueuePolicy) in gdb, we noticed about 100,000 messages in each client's
queue with old sequence numbers that correspond to 2/3 of the messages that we
never saw redelivered
- The first 1/3 or so of the messages we expected to be redelivered weren't in
those queues
- When we finally stopped one of the receiver clients, it cored (aborted), the
other receiver client died, and the qpid broker also cored
- There was a logged qpid::TransportFailure exception that happened right
before all of these crashed
Here are some of our thoughts/questions:
- We think the 1/3 of the messages that vanished might have been because the
queue filled, and the ring policy caused them to be deleted
- We think that the 2/3 of the messages we expected to be redelivered, might
not have got redelivered because the session on the new clients might have been
started before the sessions of the clients that went down with a reboot were
ended. Is there some sort of session timeout that must occur before the new
session gets these redelivered? What happens in this case?
I'm still not quite clear on what exactly your clients do.
Jeff: A receiver client subscribes to a single queue, and using a
qpid::broker::LocalQueue, gets messages, does some processing on them, and
writes the processed messages to an open temporary file. On a 15-minute
interval, the client will move the temporary file to a permanent output
directory, and then send an accept() to the broker for all the messages that
were in that file, since they have now been fully processed.
- We think the slowdown is because of the 100,000 unaccepted messages on the
front of the RingQueuePolicy's queue. We send about 200,000 ids to accept after
a 15 minute period, so for each of these messages, it will have to traverse
over the 100,000 unaccepted ids. Could this account for such a huge slowdown
and 100% cpu usage on the accept() with 200,000 ids?
Yes, it could, especially if some of those messages have been removed
from the ring queue already to make room for newer messages.
Jeff: After looking through the code and doing some debugging on the broker,
it's still not clear to me how the messages are stored. There seem to be
several deques that correspond to a single queue on the broker. Are you saying
that if a message is removed from the ring queue, the broker still maintains a
reference to that message if it was unacked? If so, is this a leak, or does the
a copy of then actual message still exist somewhere else?
Can you send accepts more frequently? Batching accepts is good to some
extent, but if you can reduce the set of in-doubt messages held by the
broker you will likely improve the performance.
Jeff: I can configure the time interval to be a bit lower. I think the
performance is only affected if the broker keeps a bunch of unacked messages
that it also never redelivers (which sounds like a bug).
The broker certainly tracks delivered messages until they are acked. If
the session they were delivered to ends without the messages being acked
they will be requeued and redelivered to the next available subscriber.
As above, I'm not clear in the case of a blade failure how these
messages from the active broker get to the standby broker from where
they can be redelivered... or is the expectation simply that since they
are durable they will be redelivered once the original active broker
recovers and becomes active again?
In the latter case, the recovery should have all the unacked messages
back on the queue ready for delivery to any available subscriber.
(Obviously in the case of a ring queue only a finite number of messages
will be recovered, as once the configured size is reached the older
messages are deleted).
The other strange thing is that I have tried to simulate this same scenario by
acquiring 100k messages and never accepting them, then continually acquiring
and then accepting batches of 200k messages, and the performance is still very
fast. The difference was that it never seems to call into the RingQueuePolicy,
since the policy pointer is null on the queue. I guess this means that most of
the work is actually done in the
RingQueuePolicy::dequeued()/RingQueuePolicy::find() - which matches the fact
that every backtrace I got was somewhere in there.
Yes, the ring queue has some undesirable inefficiencies, particularly
where you ack a message that has already been deleted on a large queue.
(That is something that we will be fixing in the not too distant future
I hope, enabled by some refactoring of the queueing code).
On a side note, I'm not sure why my queue on my attempt at simulating the
problem didn't have a policy, since I also set the queue options to have a ring
policy. Any ideas there?
Would need more detail on the steps you used.
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