CP Performance Facilities

CP provides a set of performance facilities that can be used by individual virtual machines specifically to improve their performance. The performance of a virtual machine that uses one or more of these performance facilities usually improves at the expense of reduced performance for other virtual machines, because they must compete for the use of a smaller portion of the real machine resources.

CP provides the following performance facilities (described in more detail in the sections that follow):
  • Processor dedication option. This facility can be used to reserve a real processor for the sole use of a virtual processor belonging to any virtual machine. This option can be assigned to multiple virtual machines.
  • Virtual machine multiprocessing. This option, particularly in conjunction with the dedication of real processors to virtual processors, can be used to increase the amount of work that can be done by a virtual machine. This option can be assigned to multiple virtual machines. See Virtual Machine Multiprocessing for more detailed information.
  • System scheduling control options. These options can be used to:
    • Change the maximum working set size of a virtual machine allowed in the dispatch list
    • Adjust the size of the dispatch time slice for all virtual machines
    • Allocate more or less storage to virtual machines in different transaction classes
    • Allocate more or less paging resource to heavily-paging virtual machines in different transaction classes
    • Change the maximum number of virtual machines of different transaction classes allowed in the dispatch list
    • Adjust the amount of interactive bias assigned to interactive virtual machines.
    These options affect the performance of all virtual machines. See System Scheduling Control Options for more detailed information.
  • Scheduling share option. The scheduling share option can ensure that a virtual machine has priority access to real processor, real storage, and paging resources. This option can be assigned to multiple virtual machines simultaneously. See Scheduling Share Option for more detailed information.
  • Scheduling maximum share option. The maximum share option can be used to limit virtual machines from using more than a given amount of resources. See Scheduling Maximum Share Option for more detailed information.
  • Scheduling maximum share using CPU pools. CPU pools can be used to set the maximum share of virtual CPU resources for groups of virtual machines. See Scheduling Maximum Share Using CPU Pools for more detailed information.
  • Quick dispatch option. The quick dispatch option can be used to ensure that a virtual machine does not wait in the eligible list for resources but is dispatched immediately, whenever it has work to do. See Quick Dispatch Option for more detailed information.
  • Reserved page frames option. This facility can be used to maintain a resident set of private pages for a virtual machine or shared pages for an NSS or DCSS. This option can be assigned to more than one virtual machine, NSS, or DCSS at a time. See Reserved Page Frames Option for more detailed information.
  • Locked pages option. This facility is provided to cause specific pages of a virtual machine to be permanently locked, so that no paging occurs for these pages. (In this case permanently means until the next IPL.) This facility can be used concurrently by multiple virtual machines. See Locked Pages Option for more detailed information.
  • Collaborative Memory Management Assist. The Collaborative Memory Management Assist is a machine feature that allows z/Architecture® guests with the appropriate support to exchange memory usage and status information with z/VM. For more information, see Collaborative Memory Management Assist.
  • Real channel program execution option. This facility is provided to allow V=V machines to run real channel programs, bypassing CP channel program translation. This facility can be used concurrently by multiple virtual machines. See Real Channel Program Execution Option for more detailed information.
  • Named saved systems (NSSs). This facility is provided to reduce the significant amount of CP processing that is required to IPL an operating system in a virtual machine. This facility includes the capability of sharing segments of virtual storage of the NSS among concurrently operating virtual machines on a shared read-only or read-write basis. See Named Saved Systems (NSSs) for more detailed information.
  • Saved segments. This facility enables segments of virtual storage that are not part of an NSS to be saved. It enables the virtual storage used by a virtual machine to be dynamically expanded and reduced during system operation without operator intervention and provides for the sharing of reenterable segments by concurrently operating virtual machines. See Saved Segments for more detailed information.
  • VM/VS handshaking. This facility can be used to improve the operation of VSE operating systems running in a virtual machine. It includes a facility to reduce the amount of CP processing required to handle BTAM autopolling channel programs. See VM/VS Handshaking for more detailed information.
  • Interpretive-execution facility. This processor facility eliminates much of the CP processing that would otherwise be required to simulate most privileged instructions and certain interrupts for a virtual machine by performing these functions by way of hardware. The interpretive-execution facility can be used concurrently by all logged-on virtual machines.
  • Guest wait-state interpretation capability. This capability allows a virtual processor to remain dispatched even when it enters an enabled wait state.
  • Minidisk Caching. Minidisk cache may provide performance and administrative benefits to z/VM systems. For minidisk cache, CP uses real storage as a cache for virtual I/O data. Accessing the data from electronic storage is much more efficient than accessing DASD. See Minidisk Cache for more detailed information.
  • VM Data Spaces. VM Data Spaces provide increased storage addressability and therefore can move the burden of I/O from an application to the CP paging subsystem. The use of VM Data Spaces also extends the concept of sharing data. See VM Data Spaces for more detailed information.
  • File Caching Option for CP-accessed minidisks. This option enables CP to cache its frequently used information (such as log message and logo picture files) in storage. See File Caching Option for CP-Accessed Minidisks for more detailed information.
  • Hot I/O Detection. The Hot I/O detection function prevents broken hardware from degrading performance by flooding the system with unsolicited interrupts. See Hot I/O Detection for more detailed information.
  • Virtual Disks in Storage. Virtual disks in storage are FBA minidisks allocated from host real storage instead of on real DASD, which avoids the I/O overhead. See Virtual Disks in Storage for more detailed information.
  • I/O Throttling. The rate of I/Os issued to a real device can be throttled. This is particularly helpful in environments with shared devices. The rate can be set dynamically or in the system CONFIG file. See I/O Throttling for more detailed information.
  • Enhanced QDIO Performance. QDIO virtualization technologies benefit guest operating system I/O and page management. For more information, see Enhanced QDIO Performance.
  • Parallel Access Volumes (PAV) and HyperPAV for guest I/O to minidisks. CP can take advantage of PAV or HyperPAV technology to increase the performance of guest minidisk I/O. If the DASD subsystem offers alias devices for a real volume that contains guest minidisks, CP can use those alias devices to launch multiple concurrent real I/Os against the real volume, each real I/O corresponding to a virtual I/O a guest has started against a minidisk. In this way, guest I/Os generally experience reduced real-volume queueing and, in turn, decreased response time. For more information, see Parallel Access Volumes (PAV) and HyperPAV for guest I/O to minidisks.
  • HyperPAV for the CP Paging Subsystem. CP can take advantage of HyperPAV technology to increase the performance of Paging Subsystem I/O. If the DASD subsystem offers alias devices for a real volume that contains page, spool, directory, or mapped-minidisk pool space, CP can use those alias devices to launch multiple concurrent I/O operations against the volume. This parallelism reduces volume-queuing, and, in turn, decreases response time. For more information, see HyperPAV for the Paging Subsystem.