PI62609: When coreThreads and maxThreads are the same value, CWWKE1200W m essages, which indicate a hung thread, may appear erroneously

Fixes are available

APAR status

Closed as program error.

Error description

When checking to see if a thread is possibly hung in
WebSphere
Liberty, several conditions are checked, including if the
number of threads is set to the max size of the pool, and
when
coreThreads and maxThreads are configured to be equal, this
condition is always satisfied, triggering the CWWKE1200W
messages.

While there may be a small timing window that this condition
won't occur, it may still result in the message due to other
conditions for flagging hung threads.
&#160;

Local fix

Setting the attributes coreThreads and maxThreads to different
values inside the <executor> tag in the server.xml works around
the issue.

Problem summary

****************************************************************
* USERS AFFECTED:  All users of IBM WebSphere Application      *
*                  Server Liberty                              *
****************************************************************
* PROBLEM DESCRIPTION: CWWKE1200W Hung thread message is       *
*                      logged when the thread pool is idle.    *
****************************************************************
* RECOMMENDATION:                                              *
****************************************************************
In situations where the coreThreads and maxThreads setting on
the executor element are the same (or very close), the server
may log warning messages such as the following when the thread
pool is not actually hung or deadlocked:
[5/2/16 10:40:26:448 CDT] 00000019
com.ibm.ws.threading.internal.ThreadPoolController           W
CWWKE1200W:  All threads in the Liberty default executor appear
to be hung.  Liberty automatically increased the number of
threads from 25 to 25.  However, all threads still appear to be
hung.

The logic that checks for hung thread conditions uses three
conditions:
1) The thread pool size is at maximum.
2) That the number of completed tasks has not changed over
time.
3) That the incoming buffer of tasks is not empty, indicating
that there is work needing to be done.

When the coreThreads and maxThreads are the same, the thread
pool will be maximum once the pool has created a sufficient
number of threads.  At that point condition 1 is satisfied (and
will remain satisfied until the server is stopped).

If the server is idle, then the number of completed tasks will
not change over time, satisfying condition 2.

If a new task is put on the queue of an idle thread pool at such
a time that the server checks it for hung threads, but before a
thread in the pool could complete the task, then condition 3
would also be satisfied.  This would lead to the warning message
getting printed in the logs even though the thread pool is not
hung at all.

Problem conclusion

The fix for this APAR prevents logging of the warning message
when the thread pool is not hung.

The fix for this APAR is currently targeted for inclusion in fix
pack 16.0.0.3.  Please refer to the Recommended Updates page for
delivery information:
http://www.ibm.com/support/docview.wss?rs=180&uid=swg27004980

Temporary fix

There are various ways to work around this problem:
1) Do not specify the coreThreads or maxThreads settings at all.
In general, the thread pool does a good job of estimating
workload and adjusting the thread pool settings accordingly. For
more information, see:
https://developer.ibm.com/wasdev/docs/was-liberty-threading-and-
why-you-probably-dont-need-to-tune-it/
2) Do not specify the same (or too closely similar) values for
coreThreads and maxThreads.
3) Enable some tasks that keep the thread pool from going idle -
such as file or configuration monitoring.  This will prevent the
warning message by ensuring the that the thread pool continually
completes tasks (incrementing the task count).
4) Ignore the message and use other methods to determine whether
the server's thread pool is really hung or deadlocked.

Comments

APAR Information

APAR number
PI62609
Reported component name
WAS LIBERTY COR
Reported component ID
5725L2900
Reported release
855
Status
CLOSED PER
PE
NoPE
HIPER
NoHIPER
Special Attention
NoSpecatt / Xsystem
Submitted date
2016-05-17
Closed date
2016-06-09
Last modified date
2016-06-09

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
WAS LIBERTY COR
Fixed component ID
5725L2900

Applicable component levels

R855 PSY
UP

[{"Business Unit":{"code":"BU059","label":"IBM Software w\/o TPS"},"Product":{"code":"SSD28V","label":"WebSphere Application Server Liberty Core"},"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"855","Line of Business":{"code":"LOB45","label":"Automation"}}]

Document Information

Modified date:
04 May 2022

Tips

PI62609: When coreThreads and maxThreads are the same value, CWWKE1200W m essages, which indicate a hung thread, may appear erroneously

Fixes are available

Subscribe

APAR status

Closed as program error.

Error description

Local fix

Problem summary

Problem conclusion

Temporary fix

Comments

APAR Information

APAR number

Reported component name

Reported component ID

Reported release

Status

PE

HIPER

Special Attention

Submitted date

Closed date

Last modified date

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

Fixed component ID

Applicable component levels

R855 PSY

Document Information

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