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Performance Improvements

Large Enhancements

In Summary of Key Findings this report gives capsule summaries of the performance notables in z/VM for z13.

Small Enhancements

z/VM for z13 contains one small functional enhancement that might provide performance improvement for guest operating systems that are susceptible to the repaired problem.

IPTE Interlock Alternate:

Efficiencies in handling of guest DAT-serializing instructions (IPTE, IDTE, CSG, CSPG) were added because of the potential to have a larger number of virtual CPUs defined per guest virtual machine when exploiting SMT. An improved hardware interlocking mechanism was exploited which allows the relatively long running host translation process to proceed more efficiently. When a guest DAT-serializing instruction is executed in a guest for which a host translation is also being performed, the guest instruction is intercepted as before, but the instruction is no longer automatically simulated. Instead z/VM backs up the guest PSW instruction address and gives control back to the guest to reexecute the DAT-serializing instruction. This process is referred to as IPTE redrive even though it applies to the whole family of supported DAT-serializing instructions. z/VM does still resort to simulation in exceptional circumstances, such as when instruction tracing is active, or when the guest is redriving excessively without making forward progress, or when the guest is running under a hypervisor which itself is running in a z/VM virtual machine.

With this performance enhancement, a virtual machine that issues a high number of DAT-serializing instructions should experience a reduction in simulation overhead and in turn an increase in guest utilization.

Service Since z/VM 6.3

z/VM for z13 also contains a number of changes or improvements that were first shipped as service to earlier releases. Because IBM refreshes this report only occasionally, IBM has not yet had a chance to describe the improvements shipped in these PTFs.

VM64460: Minidisk Cache (MDC) would stop working in some circumstances. The failure was due to MDC PTEs not being reclaimed in a timely way. The PTF repairs the problem.

VM65425: COPYFILE exhibited poor performance when the source file resided in the CMS Shared File System. A change to DMSCPY to use larger read buffers repaired the problem.

VM65426: A system hang was possible because HCPHPH inadvertently acquired PTIL and shadow table lock on the SYSTEM VMDBK. The error was introduced in the PTF for VM64715. The PTF for VM65426 repairs the problem.

VM65476: Response output from the NETSTAT TELNET and NETSTAT CONN commands displayed slowly if file ETC HOSTS was not found on any CMS accessed minidisk or SFS directory. The PTF repaired the problem.

VM65518: The MAPMDISK REMOVE function would hang if the system were under paging load. This problem was the result of a defect in the PTF for APAR VM65468. The PTF for VM65518 repairs the problem.

VM65549: Page reads from EDEV paging devices were not being counted. The PTF solves the problem.

VM65655: Virtual storage release processing was slow because an IPTE was being done on an invalid PTE. The PTF repairs the problem.

VM65683: The Control Program's spin lock manager issues Diag x'9C' to PR/SM when the acquiring CPU finds the lock held and detects that the holder could be expedited by issuing the Diagnose against the holding CPU. A defect in the loop that identifies the lock holders caused the Control Program to issue excessive numbers of these Diag x'9C' instructions. The PTF repairs the problem.

VM65696: This moves the scheduler lock to its own cache lines. The lock's SYNBK is now alone on its own line, as is its SYNBX.

Miscellaneous Repairs

IBM continually improves z/VM in response to customer-reported or IBM-reported defects or suggestions. In z/VM for z13 the following small improvements or repairs are notable:

Long LOGOFF Times: LOGOFF can take a long time in the presence of parked logical CPUs. The repair solves the problem.

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