CODEWITHSUNDEEP
ibm-mq
Earle Ake
Earle Ake

Reputation: 21

mq slow persistent message reading

I am trying to track down an issue where a client can not read messages as fast as they should. Persistent messages are written to a queue. At times, the GET rate is slower than the PUT rate and we see messages backing up.

Using tcpdump, I see the following:

MQGET: Convert, Fail_If_Quiescing, Accept_Truncated_Msg, Syncpoint, Wait
Message is sent
Notification
MQCMIT
MQCMIT_REPLY

In analyzing the dump, sometimes I see the delta between the MQCMIT and MQCMIT_REPLY be in the 0.001 second timeframe and I also see it in the 0.1 second timeframe. It seems like the 0.1 sec delay is slowing the message transfer down. Is there anything I can do to decrease the delta between the MQCMIT and MQCMIT_REPLY? Should the client be reading multiple messages before the MQCMIT is sent?

This is MQ 8.0.0.3 on AIX 7.1.

Upvotes: 2

Views: 1359

Answers (1)

Yuri Steinschreiber
Yuri Steinschreiber

Reputation: 2698

The most straightforward way to increase message throughput on the receiving side is to batch MQGET operations. That is, do not issue MQCMIT for every MQGET, but rather after a number of MQGET operations. MQCMIT is the most expensive operation for persistent messages since it involves forcing log writes on the queue manager, and therefore suffers disk I/O latency. Experiment with the batch size - I often use 100, but some applications can go even higher. Too many outstanding MQGET operations can be problematic since they keep the transaction running for much longer time and prevent the log switching.

And of course you can check if your system overall tuning is satisfactory. You might have too long a latency between your client and queue manager, or your logs may reside on a slow device, or the logs may share the device with the queue files or an otherwise busy filesystem.

Upvotes: 2

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