IT23790: Applications using asynchronous messaging may trigger high CPU usage in amqzlaa0

APAR status

• Closed as program error.

Error description

• When using asynchronous messaging (for example the AMQSCBF
  sample supplied with MQ), the MQCB callback may enter an
  infinite
  loop.
  
  This results in no messages being consumed by the application.
   The
  amqzlaa0 process may be seen to be using a lot of CPU.
  

Local fix

• If the application uses an MQMD version 2 then this should work
  around the problem.
  

Problem summary

• ****************************************************************
  USERS AFFECTED:
  Users of applications that use asynchronous consume with a
  version 1 MQMD and MQCBD_FULL_MSG_LENGTH set and which consume
  messages that have a version 2 MQMD with MsgFlags field set may
  be affected by this problem.
  
  
  Platforms affected:
  MultiPlatform
  
  ****************************************************************
  PROBLEM DESCRIPTION:
  An application was configured to consume messages using
  asynchronous consume with a version 1 MQMD and
  MQCBD_FULL_MSG_LENGTH in the callback descriptor.  The
  application attempted to consume a message that used an MQMD
  version 2 with the MsgFlags field set.  A bug in the IBM MQ
  asynchronous consume code caused it to enter a tight loop where
  it continually uses a smaller buffer than necessary to consume
  the message from the queue.
  
  This behaviour is seen where the size of the message is close to
  a 4096 byte boundary and where it is necessary to add an MQMDE
  to the message.  The behaviour may therefore appear to occur at
  random.
  

Problem conclusion

• The code was modified to ensure that the buffer provided to
  consume the message is large enough in the case where it is
  necessary to add an MQMDE to the message.
  
  ---------------------------------------------------------------
  The fix is targeted for delivery in the following PTFs:
  
  Version    Maintenance Level
  v8.0       8.0.0.9
  v9.0 LTS   9.0.0.4
  
  The latest available maintenance can be obtained from
  'WebSphere MQ Recommended Fixes'
  http://www-1.ibm.com/support/docview.wss?rs=171&uid=swg27006037
  
  If the maintenance level is not yet available information on
  its planned availability can be found in 'WebSphere MQ
  Planned Maintenance Release Dates'
  http://www-1.ibm.com/support/docview.wss?rs=171&uid=swg27006309
  ---------------------------------------------------------------
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

Fix information

• Fixed component name

  IBM MQ BASE MP

• Fixed component ID

  5724H7251

Applicable component levels