IBM Support


A fix is available


You can track all active APARs for this component.


APAR status

  • Closed as program error.

Error description

  • Using a dedicated HiperSockets device in a virtual machine
    running Linux exploiting QDIO Enhanced Buffer State Management
    (QEBSM), may experience slow performance in a very highly
    contested LPAR to LPAR communication environment.  This is an
    environment where multiple HiperSockets data transmissions occur
    simultaneously to the same set of QDIO queues.  It's this
    contested environment which increases the likelihood of a
    HiperSockets device presenting a short busy condition on a
    Signal Adapter (SIGA) Instruction issued by a program (the Linux
    Guest in this case).  It is the occurrence of a HiperSockets
    short busy which opens an error window that may cause the
    problem to occur.
    Running with QIOASSIST OFF increases the likelihood
    of seeing this problem.

Local fix

Problem summary

  • ****************************************************************
    * USERS AFFECTED: All customers using a dedicated HiperSockets *
    *                 device with QEBSM in a virtual machine.      *
    * PROBLEM DESCRIPTION:                                         *
    * RECOMMENDATION: APPLY PTF                                    *
    The performance impact is caused by z/VM's processing of a QEBSM
    SIGA Instruction.  If z/VM handles the instruction, it may not
    reflect the short busy condition correctly to a guest.  This
    could occur when QIOASSIST is either turned off for a QEBSM
    Eligible HiperSockets device or if HiperSockets firmware taps
    z/VM on the shoulder to complete a SIGA when the assist is on.
    Not reflecting a short busy correctly will cause Linux's QDIO
    queue processing to get out of SYNC with the hardware, resulting
    in a drastic decrease in network performance from the point of
    failure on.
    Running with QIOASSIST ON, the chance of running into this
    problem is small.  With the assist on, HiperSockets firmware
    will process 99.9 percent of all QEBSM SIGA Instructions
    directly without z/VM involvement.  If a short busy condition is
    encountered while HiperSockets firmware is processing the
    instruction, it will be correctly presented to the guest.  Only
    the less than one percent of the cases where z/VM handles the
    SIGA Instruction itself is there a exposure for this problem.
    Given a short busy condition is not a typical event, the chances
    of z/VM running into the condition is very small with QIOASSIST
    ON.  The chance will increase considerably if QIOASSIST is
    turned off for the guest or system.  Turning off the assist will
    cause z/VM to execute all SIGA Instructions on behalf of the
    guest, thereby increasing the likelihood of running into the
    performance problem.

Problem conclusion

  • Additional logic is added to z/VM's virtual SIGA Instruction
    logic to properly reflect a HiperSockets short busy condition
    for a guest exploiting a QDIO Enhanced Buffer State Management
    (QEBSM) network connection.

Temporary fix


APAR Information

  • APAR number


  • Reported component name

    VM CP

  • Reported component ID


  • Reported release


  • Status


  • PE




  • Special Attention

    NoSpecatt / Xsystem

  • 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:

    UM35032 UM35033 UM35034 UM35035



Fix information

  • Fixed component name

    VM CP

  • Fixed component ID


Applicable component levels

  • R540 PSY UM35032

       UP17/02/15 I 1000

  • R620 PSY UM35033

       UP17/02/15 I 1000

  • R630 PSY UM35034

       UP17/02/15 P 1701

  • R640 PSY UM35035

       UP17/02/15 P 1701

Fix is available

  • Select the PTF appropriate for your component level. You will be required to sign in. Distribution on physical media is not available in all countries.

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Document Information

Modified date:
25 August 2017