VM65730: SET RDEV CAUSES OVERHEAD/SUSPEND TIME ON PROCESSOR

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• SET RDEV command for the maximim range of 255 unknown devices
  causes the I/O subsystem to discover the location of the
  devices.  During the
  discovery the STSCH instruction can be invoked repeatedly.  The
  STSCH instruction can cause LPAR Overhead and Processor Suspend
  time on the processor invoking the STSCH instruction. This can
  lead to poor performance and guests to stall if they are being
  dispatched on the processor experiencing the Overhead and
  Suspend time.
  

Local fix

• Avoid the SET RDEV command for the maximum range of 255.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users using SET RDEVICE, dynamic I/O,    *
  *                 and VARY <real device> commands to bring     *
  *                 new or NOTACCEPTED devices into the          *
  *                 configuration                                *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  ****************************************************************
  * RECOMMENDATION: APPLY PTF                                    *
  ****************************************************************
  Devices that are excluded from the z/VM I/O configuration with
  DEVICES NOTACCEPTED statements can be added to the configuration
  dynamically after IPL using the SET RDEVICE command. This
  command uses a Store Subchannel (STSCH) instruction to search
  for the subchannel identifiers of the designated device numbers.
  This can cause a large number of STSCH instructions to be
  executed, depending on the subchannel identifiers of the new
  devices. In large configurations with thousands of devices, it
  is not unusual for z/VM to issue several thousand STSCH
  instructions to find the subchannel identifier for each new
  device. If a range of devices is being added with a single SET
  RDEVICE command, several million STSCH instructions could be
  executed. This in turn can consume several seconds of processor
  time. Because of the way that time is accounted, performance
  analysis tools such as z/VM Performance Toolkit will report it
  as Suspend time in, for example, the "%Susp" field of FCX304,
  "Processor Log Screen - PRCLOG".
  

Problem conclusion

• The frequency of STSCH instruction execution has been reduced by
  keeping track of the most recent subchannel identifier that is
  found during SET RDEVICE command processing and starting the
  next search with the next higher value. If a range of devices is
  specified, this takes advantage of the natural proximity of
  their subchannel identifiers, decreasing CPU consumption and
  eliminating excess Suspend time reporting.
  

Temporary fix

• *********
  * HIPER *
  *********
  FOR RELEASE VM/ESA CP/ESA R640 :
  PREREQ: VM66105
  CO-REQ: NONE
  IF-REQ: NONE
  FOR RELEASE VM/ESA CP/ESA R710 :
  PREREQ: NONE
  CO-REQ: NONE
  IF-REQ: NONE
  

Comments

APAR Information

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

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

  UM35368 UM35369

Modules/Macros

• HCPIID   HCPSYS   HCPSYSCM
  

Fix information

• Fixed component name

  VM CP

• Fixed component ID

  568411202

Applicable component levels

• R640 PSY UM35368

     UP19/03/06 P 1901 ¢

• R710 PSY UM35369

     UP19/03/06 P 1902 ¢

Fix is available

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