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Summary of Key Findings

This section summarizes the performance evaluation of z/VM 4.4.0. For further information on any given topic, refer to the page indicated in parentheses.

Performance Changes 

z/VM 4.4.0 includes a number of performance improvements and changes that affect VM performance management (see Changes That Affect Performance):

  • Performance Improvements
    • Scheduler Lock Improvement
    • Queued I/O Assist
    • Dispatcher Detection of Long-Term I/O
    • Virtual Disk in Storage Frames Can Now Reside above 2G
    • z/VM Virtual Switch
    • TCP/IP Stack Improvements
    • TCP/IP Device Layer MP Support

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

The most notable performance management change is the introduction of the Performance Toolkit for VM, which will, in subsequent releases, replace RTM and VMPRF.

Migration from z/VM 4.3.0  Regression measurements for the CMS environment (CMS1 workload) indicate that the performance of z/VM 4.4.0 is slightly better than z/VM 4.3.0. CPU time per command decreased by about 0.4% due to a 7% CPU time reduction in the TCP/IP VM stack virtual machine.

CPU usage of the TCP/IP VM stack virtual machine has been reduced significantly. CPU time reductions ranging from 5% to 81% have been observed. The largest improvement was for the CRR workload, which represents webserving workloads (see Performance Improvements and TCP/IP Stack Performance Improvements).

New Functions 

With z/VM 4.4.0, the timer management functions no longer use the scheduler lock but instead make use of a new timer request block lock, thus reducing contention for the scheduler lock. Measurement results of three environments that were constrained by scheduler lock contention showed throughput improvements of 8%, 73%, and 270% (see Scheduler Lock Improvement and Linux Guest Crypto on z990).

The z/VM support for the Queued I/O Assist provided by the IBM eServer zSeries 990 (z990) can provide significant reductions in total system CPU usage for workloads that include guest operating systems that use HiperSockets or that add adapter interruption support for OSA Express and FCP channels. CPU reductions ranging from 2 to 5 percent have been observed for Linux guests running HiperSockets workloads and from 8 to 18 percent for Gigabit Ethernet workloads (see Queued I/O Assist).

The z/VM Virtual Switch can be used to eliminate the need for a virtual machine to serve as a TCP/IP router between a set of virtual machines in a VM Guest LAN and a physical LAN that is reached through an OSA-Express adapter. This can result in a significant reduction in CPU time. Decreases ranging from 19% to 33% were observed for the measured environments when a TCP/IP VM router was replaced with a virtual switch. Decreases ranging from 46% to 70% were observed when a Linux router was replaced with a virtual switch (see z/VM Virtual Switch).

With TCP/IP 440, support has been added to allow device-specific processing to be done on virtual processors other than the base processor used by the remaining stack functions. CP can then dispatch these on separate real processors if they are available. This can increase the rate of work that can be handled by the stack virtual machine before the base processor becomes fully utilized. For the measured cases, throughput changes ranging from a 2% decrease to a 24% improvement were observed (see TCP/IP Device Layer MP Support).

Additional Evaluations 

Measurements are shown that illustrate the high SSL transaction rates that can be sustained on z990 processors by Linux guests through the use of z990 cryptographic support (see Linux Guest Crypto on z990).

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