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Summary of Key Findings

This section summarizes the performance evaluation of z/VM 4.2.0. For further information on any given topic, refer to the page indicated in parentheses.

Performance Changes 

z/VM 4.2.0 includes a number of performance enhancements (see Performance Improvements). In addition, some changes were made that affect VM performance management (see Performance Management):

  • Performance Improvements
    • Fast CCW Translation and Minidisk Caching for 64-bit DASD I/O
    • Block Paging Improvement
    • DDR LZCOMPACT Option

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

Migration from z/VM 4.1.0 

Regression measurements for the CMS environment (CMS1 workload) and the VSE guest environment (DYNAPACE workload) indicate that the performance of z/VM 4.2.0 is equivalent to z/VM 4.1.0 and that the performance of TCP/IP Level 420 is equivalent to TCP/IP Level 410.

New Functions 

z/VM 4.2.0 provides support for HiperSockets, now available on z/900 and z/800 processors. HiperSockets provides a high-bandwidth communications path within a logical partition (LPAR) and between LPARs within the same processor complex. HiperSockets support is enabled by APARs VM62938 and PQ51738. In addition, VM63034 is recommended. Measurement results using TCP/IP VM (see HiperSockets and VM Guest Lan Support) and using Linux guests (see Linux Connectivity Performance) show that HiperSockets provides excellent performance that compares well with existing facilities such as IUCV and virtual CTC that VM provides for communication within a single VM system.

z/VM 4.2.0 introduces VM Guest LAN, a facility that allows a VM guest to define a virtual HiperSockets adapter and connect it with other virtual HiperSockets adapters on the same VM system to form an emulated LAN segment. This allows for simplified configuration of high speed communication paths between large numbers of virtual machines. Measurement results using TCP/IP VM (see HiperSockets and VM Guest Lan Support) and using Linux guests (see Linux Connectivity Performance) indicate that this support performs well over a wide range of workloads and system configurations.

The CCW translation fast path and minidisk caching have now been extended to include 64-bit DASD I/O, resulting in reduced processing time and I/Os. Which cases are eligible and the degree of improvement are equivalent to what is already experienced with 31-bit I/O. (Exception: FBA devices do have fast CCW translation for 64-bit I/O but not MDC support.) CP CPU time decreases, ranging from 32% to 38%, were observed for the measured workload (see 64-bit Fast CCW Translation).

Guest support for the FICON channel-to-channel adapter is provided by z/VM 4.2.0 with APAR VM62906. Throughput of bulk data transfer was measured to be over twice that obtained using an ESCON connection, while CPU usage per megabyte transferred was similar (see Guest Support for FICON CTCA).

Measurements indicate that the new, 64-bit capable PFAULT service provides performance benefits that are comparable to the existing (31-bit) PAGEX asynchronous page fault service (see 64-bit Asynchronous Page Fault Service (PFAULT)).

In most cases, use of the LZCOMPACT option will reduce the length of tape required to hold a DDR dump relative to using the existing COMPACT option. Decreases ranging from 1% to 28% were observed (see DDR LZCOMPACT Option).

TCP/IP level 420 provides an IMAP server. Measurement results show that one IMAP server running on z/900 hardware can support over 2700 simulated IMAP users with good performance (see IMAP Server).

Additional Evaluations 

Measurement results indicate that the usual minidisk cache tuning guidelines apply to the case of Linux guests doing I/O to a VM minidisk. That is, MDC is highly beneficial when most I/Os are reads but causes additional overhead, and therefore should be turned off, for minidisks where many of the I/Os are writes (see Linux Guest DASD Performance).

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