IBM Support


A fix is available


You can track all active APARs for this component.


APAR status

  • Closed as new function.

Error description

  • Simultaneous multithreading (SMT) and increased CPU scalability

Local fix

  • N/A

Problem summary

  • ****************************************************************
    * USERS AFFECTED: Installations running z/VM on an IBM z13     *
    *                 that want to utilize Simultaneous            *
    *                 Multithreading (SMT) support for IFL         *
    *                 processors (which supports up to 32 logical  *
    *                 cores) or to utilize up to 64 logical        *
    *                 single threaded cores.                       *
    * PROBLEM DESCRIPTION: SMT and up to 64 core support           *
    *                      required in z/VM for IBM z13 (z13)      *
    *                      D/T2964.                                *
    * RECOMMENDATION: APPLY PTF                                    *
    SMT and up to 64 core support required in z/VM for IBM z13
    (z13) D/T2964.

Problem conclusion

Temporary fix


  • With the PTF for APAR VM65586, z/VM provides host exploitation
    support for the simultaneous multithreading (SMT) facility on
    the IBM z13, which will enable z/VM to dispatch work on up to
    two threads (logical CPUs) of an IFL processor core. z/VM
    multithreading support is enabled only for IFL processors in a
    LINUX only mode or z/VM mode logical partition.  This may
    result in increased work throughput per core from more
    efficient use of shared core resources.
    Additionally, z/VM will now support up to 64 logical processors
    on the IBM z13:
    - 64 cores with multithreading disabled
    - 32 cores (up to 2 threads per core) with multithreading
    z/VM continues to support up to 32 logical processors on
    prior machines.
    SMT support is available on a z/VM system only if the facility
    is installed on the hardware and enabled on the z/VM system
    with the MULTITHREADING system configuration statement. The
    MULTITHREADING statement is optional, and multithreading is
    disabled if the statement is omitted.  z/VM enablement of
    multithreading requires that z/VM is configured to run with
    the HiperDispatch vertical polarization mode enabled and with
    the dispatcher work distribution mode set for reshuffle. If
    the system configuration file changes either of these
    conditions so they are not set correctly for multithreading,
    the system will come up with multithreading disabled. SMT
    cannot be enabled or disabled without changing the system
    configuration file and issuing a re-IPL. The new QUERY
    MULTITHREAD (MT) command can be used to discover if
    SMT is enabled on a system.
    z/VM host support for SMT does not virtualize threads for
    guest exploitation.
    Performance Toolkit support for SMT requires APAR VM65529.
    Stand-alone dump support for SMT requires APARs VM65676 and
    To support this APAR, new editions of the following publications
    are available:
       * z/VM V6.3 CP Commands and Utilities Reference, SC24-6175-06
          Information on new commands (INDICATE MT, QUERY MT,
          VARY CORE) and changed commands
       * z/VM V6.3 CP Messages and Codes, GC24-6177-06
          Information on new messages and Abend codes
       * z/VM V6.3 CP Planning and Administration, SC24-6178-07
          Information on the new MULTITHREADING statement,
          changed SRM statement, new accounting record
          (type F), and changed accounting records
       * z/VM V6.3 CP Programming Services, SC24-6179-06
          Information on new programming interfaces
       * z/VM V6.3 Diagnosis Guide, GC24-6187-04
          Information on new trace codes
       * z/VM V6.3 General Information, GC24-6193-08
       * z/VM V6.3 License Information, GC24-6200-10
       * z/VM V6.3 Migration Guide, GC24-6201-06
          Information on the new monitor record,  Domain 5
          Record 20, and several changed monitor records
       * z/VM V6.3 Performance, SC24-6208-06
          Information on the new monitor record
    For more information about the publication changes,
    refer to the Summary of Changes section located near
    the beginning of each publication.
    See for the online version
    of the updated publications.
    Note: z/VM CPU pools provide a mechanism for limiting the CPU
    resources consumed by a group of virtual machines to a specific
    capacity. In an environment without SMT, these capacities are
    enforced in terms of a number of cores. In an SMT environment,
    these capacities are enforced in terms of a number of threads.
    Consequently, it might be necessary to increase the capacities
    of CPU pools, in order to provide adequate resource to CPU
    pool members.
    Note:  If you are using a VM mode partition with multithreaded
    logical IFLs then, in order for z/VM to use these IFLs, guests
    need to have virtual IFLs defined.  For information on how to
    define virtual IFLs, see Running Guest Operating Systems,
    Chapter 1 - "Fundamentals of Guest Support in z/VM", section
    "Specialty Engine Support".

APAR Information

  • APAR number


  • Reported component name

    VM CP

  • Reported component ID


  • Reported release


  • Status


  • PE




  • Special Attention

    YesSpecatt / New Function / 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:

    UM34551 UM34552


    ***This field was truncated.  To obtain the full apar record, please contact your local support center.***

Fix information

  • Fixed component name

    VM CP

  • Fixed component ID


Applicable component levels

  • RA63 PSY UM34551

       UP15/09/22 P 1502

  • R630 PSY UM34552

       UP15/03/12 P 1502

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:
11 December 2017