A fix is available
APAR status
Closed as program error.
Error description
Contention between Deferred write processing and Readahead processing is causing both delays to deferred write processing, and an increase in requests for readahead processing. This in turn has resulted in an increase in storage required by readahead, and in this instance, caused storage exhaustion . In MSTR joblog,the following messages are displayed: 01.28.38 S0129303 CSQY222E +CSQ1 CSQSCTL Queue manager is critically short of local storage - take action ... 01.33.44 IEA794I SVC DUMP HAS CAPTURED: DUMPID=056 REQUESTED BY JOB (CSQ1MSTR) DUMP TITLE=CSQ1,ABN=5C6-00E20003,U=SYSOPR ,C=R3600.710.BUFF-CSQP1RAH,M=CSQGFRCV,LOC=CSQSLD1 .CSQSVBK +00000C44 Additional Symptom(s) Search Keyword(s): performance cpu I/O MMRE
Local fix
n/a
Problem summary
**************************************************************** * USERS AFFECTED: All users of WebSphere MQ for z/OS Version 7 * * Release 1 Modification 0. * **************************************************************** * PROBLEM DESCRIPTION: Various problems encountered when queue * * readahead is enabled (for example, when * * using QREP). * * * * Problems can include: * * - High CPU utilisation by the queue * * manager address space, in some cases * * leading to abend S071-20 and abnormal * * queue manager termination * * - High number of IO requests to the * * pageset datasets * * - Increase in queue manager storage * * associated with MMRE control blocks, * * potentially leading to a Short On * * Storage condition and abnormal * * queue manager termination * **************************************************************** * RECOMMENDATION: * **************************************************************** When using readahead, getting tasks queue requests to the readahead task so that it can bring in the pages required by the getter ahead of them being requested, and consequently reduce pageset i/o delays in the getting task. However, if the readahead task is delayed it can fall behind the getting task. This leads to an increase in the number of requests queued to the readahead task, including multiple requests for the same pages. When processing the requests, the large number of duplicated page requests can cause CSQIRAHP to loop scanning a list of pages for processing, leading to high cpu on this SRB. When the readahead task finishes processing the requests, or finds a request requiring IO to satisfy, it will attempt to load those pages, leading to further cpu cost if the pages are already in buffers, or IO requests if they are not. However the majority of these requests are for pages no longer needed by the getting tasks. If requests are being queued to the readahead task faster than it can process them, there can be a considerable delay in MMRE blocks associated with completed IOs being freed. This leads to an increase in the storage required for MMREs, due to the MMREs for completed IOs not being available for reuse by new IO requests.
Problem conclusion
CSQIRAHP is changed to detect requests that are no longer current, and avoid doing unnecessary processing and IO for such requests. In addition CSQIMGE9 is changed to only queue a new request for a given page to the readahead task if that page is not already in the process of being loaded by the readahead task. 100Y CSQIMGE9 CSQIRAHP CSQPMTRM CSQP1GET CSQP1RAH
Temporary fix
********* * HIPER * *********
Comments
APAR Information
APAR number
PI52549
Reported component name
WMQ Z/OS V7
Reported component ID
5655R3600
Reported release
100
Status
CLOSED PER
PE
NoPE
HIPER
YesHIPER
Special Attention
NoSpecatt / Xsystem
Submitted date
2015-11-16
Closed date
2015-12-14
Last modified date
2016-02-01
APAR is sysrouted FROM one or more of the following:
APAR is sysrouted TO one or more of the following:
UI33764
Modules/Macros
CSQIMGE9 CSQIRAHP CSQPMTRM CSQP1GET CSQP1RAH
Fix information
Fixed component name
WMQ Z/OS V7
Fixed component ID
5655R3600
Applicable component levels
R100 PSY UI33764
UP16/01/08 P F601 ¢
Fix is available
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Document Information
Modified date:
01 February 2016