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Summary of Key Findings

This section summarizes the performance evaluation of z/VM 5.1.0. For further information on any given topic, refer to the page indicated in parentheses.

Performance Changes 

z/VM 5.1.0 includes a number of performance improvements, performance considerations, and changes that affect VM performance management (see Changes That Affect Performance):

  • Performance Improvements
    • Contiguous Frame Management Improvements
    • Improved Management of Idle Guests with Pending Network I/O
    • No Pageable CP Modules

  • Performance Considerations
    • Preferred Guest Support
    • 64-bit Support
    • FBA-emulation SCSI DASD CPU Usage
    • TCP/IP VM IPv6 Performance

  • Performance Management
    • Monitor Enhancements
    • Effects on Accounting Data
    • VM Performance Products

Migration from z/VM 4.4.0  Regression measurements comparing z/VM 4.4.0 and z/VM 5.1.0 showed performance results that are equivalent within run variability. The following environments were evaluated: CMS (CMS1 workload), Linux connectivity, and TCP/IP VM connectivity.

New Functions 

z/VM 5.1.0 now supports up to 24 processors. CP's ability to effectively utilize additional processors is highly workload dependent. For example, CMS-intensive workloads typically cannot make effective use of more than 8-12 processors before master processor serialization becomes a bottleneck. A Linux webserving workload was used to see how well CP can handle a workload that causes little master processor serialization as the number of real processors is increased to 24. LPAR processor capping was used to hold total processing power constant so as to be able to observe just n-way effects rather than a combination of n-way and large system effects. The results show that, for this workload, CP can make effective use of all 24 processors. The usual decrease in efficiency with increasing processors due to increased MP locking was observed. On a 24-way, for example, total CPU time per transaction increased 32% relative to the corresponding 16-way measurement (see 24-Way Support).

The FBA-emulation Small Computer System Interface (SCSI) support provided by z/VM 5.1.0 has much higher CPU requirements than either dedicated Linux SCSI I/O or traditional ECKD DASD I/O. This should be taken into account when deciding when to use this support (see Performance Considerations and Emulated FBA on SCSI).

Measurement results indicate that there are some cases where performance can be degraded when communicating between TCP/IP VM stack virtual machines using Internet Protocol Version 6 (IPv6), or using IPv4 over IPv6-capable devices, as compared to IPv4 (see Performance Considerations and Internet Protocol Version 6 Support). Similarly, some reduction in performance was observed for IPv6 relative to IPv4 for the case of Linux-to-Linux connectivity via the z/VM Virtual Switch using the Layer2 transport mode (see Virtual Switch Layer 2 Support).

The z/VM Virtual Switch now supports the Layer2 transport mode. 1 The new Layer2 support shows performance results that are similar to the Layer3 support that was provided in z/VM 4.4.0. In most measured cases, throughput was slightly improved, while total CPU usage was slightly degraded (see Virtual Switch Layer 2 Support).

Additional Evaluations 

A series of measurements was obtained to evaluate a number of z990 guest crypto enhancements. These results provide insight into the performance of 1) the PCIXCC card relative to the PCICA card, 2) VM's shared cryptographic support for Linux guests compared to the new dedicated cryptographic support, 3) the effect of multiple Linux guests on cryptographic performance with shared queues, 4) the effect of different ciphers on Linux SSL performance, and 5) guest versus native performance for ICSF test cases and an SSL workload on z/OS. For all measurements with multiple guests, throughput was limited by either total system processor utilization or the capacity of the available cryptographic cards (see z990 Guest Crypto Enhancements).


Requires z/VM 5.1.0 with APAR VM63538 and TCP/IP 5.1.0 with PQ98202, running on z890 or z990.

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