Channel initiator storage configuration from IBM MQ 9.3.1

The channel initiator typically uses much less 64-bit storage than the queue manager. However, from IBM® MQ 9.3.1 the usage has increased. In addition to 64-bit storage, you should allow the channel initiator to use all available 31-bit storage by specifying REGION=0M on the queue manager JCL.

Common storage

The channel initiator typically requires ECSA usage of up to 160KB.

31-bit private storage

The 31-bit storage available to the channel initiator limits the number of concurrent connections the CHINIT can have.

Every channel uses approximately 170KB of extended private region in the channel initiator address space. For message channels, for example, sender or receiver channels, storage is increased by message size if messages larger than 32KB are transmitted. This increased storage is freed when:

A sending or client channel requires less than half the current buffer size for 10 consecutive messages.
A heartbeat is sent or received.

The storage is freed for reuse within the Language Environment, however, the storage is not seen as free by the z/OS® virtual storage manager. This means that the upper limit for the number of channels is dependent on message size and arrival patterns, and on limitations of individual user systems on extended private region size.

The upper limit on the number of channels is likely to be approximately 9000 on many systems because the extended region size is unlikely to exceed 1.6GB.

The channel initiator trace is written to a data space. The size of the data space storage, is controlled by the TRAXTBL parameter. See ALTER QMGR.

64-bit private storage

The MEMLIMIT parameter of the channel initiator JCL defines the maximum amount of 64-bit storage available. 2 GB of storage, MEMLIMIT=2 GB, is the minimum value you should use. Depending on your configuration significantly more might be required.

You should specify a sensible MEMLIMIT value rather than MEMLIMIT=NOLIMIT to prevent potential problems. If you specify NOLIMIT or a very large value, then there is the potential to use up all of the available z/OS virtual storage, leading to paging in your system. When increasing the value of MEMLIMIT you should discuss the new setting with your z/OS system programmer in case there is a system-wide limit on the amount of on storage that can be used.

If you have a large value for MEMLIMIT you might need to increase the size of your dump datasets as more data is captured in a dump.

There are two users of 64-bit storage in the channel initiator: SMF and server-connection channels.

SMF

If enabled, SMF class 4 accounting, or statistics, require 64-bit storage. A minimum of 256MB storage is required. If sufficient storage is not available, the channel initiator issues the CSQX124E message and class 4 accounting and statistics are not available.

Server-connection channels

From IBM MQ 9.3.1 server-connection channels allocate message buffers in 64-bit storage, if they are transferring messages larger than 32 KB in size.

These buffers are freed if the channels require less than half the current buffer size for 10 consecutive messages, or a heartbeat is sent or received.

The value of MEMLIMIT sets an upper limit on how many concurrent server-connection channels can run. You should use a minimum value of MEMLIMIT=2G to ensure that the same number of channels can run as in earlier versions of IBM MQ 9.3.1, as well as providing some capacity for growth.

You can calculate an approximate value for MEMLIMIT by working out the peak maximum number of concurrently active server-connection channels, and for those channels the maximum message size you expect them to transfer. You should use MEMLIMIT=2GB as a starting point and round up.

For example, if you set the maximum number of concurrent server-connection channels to be 2,000 and each channel to have a maximum message size of 1MB, then server-connection channels are using a maximum of just under 2GB of 64-bit storage. As this is very close to 2GB then you should round up to MEMLIMIT=3G.