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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 6.3.

Small Enhancements

z/VM 6.3 contains several small functional enhancements that might provide performance improvements for guest operating systems that exploit the improvements. IBM did no measurements on any of these release items. The list below cites the items for only completeness' sake.

FCP Data Router: In z196 GA 2 and later, the FCP adapter incorporates a "data mover" hardware function which provides direct memory access (DMA) between the FCP adapter's SCSI interface card and System z memory. This eliminates having to stage the moving data in a buffer located on and owned by the FCP adapter. A guest operating system indicates its readiness to exploit this hardware by enabling the Multiple Buffer Streaming Facility when establishing its QDIO queues.

z/VM 6.3 lets guest operating systems enable the Multiple Buffer Streaming Facility. Operating systems capable of exploiting the feature will see corresponding improvements.

HiperSocket Completion Queue: Traditional System z HiperSockets messages are transmitted synchronously. The sending CPU is blocked until the buffer can be copied. Success or failure is returned in the condition code of the SIGA instruction.

In z196 GA 2 and later, the HiperSockets facility is enhanced so that an exploiting guest can arrange not to be blocked if there would be a delay in copying the transmit buffer. Instead, the guest can enable the completion queue facility and be notified by condition code and later interrupt if the transmission has been queued for completion later.

z/VM 6.3 lets guest operating systems exploit HiperSockets completion queues if the underlying machine contains the feature. Operating systems capable of exploiting the feature will see corresponding improvements.

Access-Exception Fetch/Store Indication Facility: This facility provides additional information in the Translation-Exception Identifier (TEID) when a storage access is denied. z/VM 6.3 lets guest operating systems use the new facility. Guests able to use the new information in the TEID might be able to improve how they run on z/VM, and might consequently experience performance improvements.

Service Since z/VM 6.2

z/VM 6.3 also contains a number of changes or improvements that were first shipped as service to earlier releases. Because IBM refreshes this report at only z/VM release boundaries, IBM has not yet had a chance to describe the improvements shipped in these PTFs.

VM65085: z/VM was changed so that when it is the destination system in a live guest relocation, it throttles the number of page handling tasks it stacks. This helps prevent LGR work from getting in the way of system work unrelated to relocations.

VM65156: z/VM was changed so that if the guest's path mask is missing a path known by z/VM actually to exist, the channel program will not necessarily bypass MDC.

VM65007: As part of the zEC12 compatibility PTF, z/VM enabled support for the Local TLB Clearing Facility. This facility lets guest operating systems issue IPTE or IDTE instructions with the Local Clearing Control (LC) bit on. Guests able to meet the LC requirements specified in the System z Principles of Operation might see performance improvements. The facility is available on the zEC12 machine family.

VM65115: This repaired D6 R27 MRIODQDD so that the RDEV field was reliable.

VM65168: CP's calculations for paging device service time and paging device MLOAD were not correct. This caused CP to favor some paging devices over others. This could result in some devices being underutilized while others are overutilized.

VM65309: Under some conditions z/VM failed to drive ordinary user minidisk I/O on a z/VM-owned HyperPAV alias, even though z/VM owned the base device and the HyperPAV alias device.

Miscellaneous Repairs

IBM continually improves z/VM in response to customer-reported or IBM-reported defects or suggestions. In z/VM 6.3, the following small improvements or repairs are notable:

Prefix-LRE Support: A prefix-LRE CCW will no longer cause an abort of CCW fast-trans, nor will it disqualify the channel program for Minidisk Cache (MDC). Certain Linux distributions' DASD driver recently changed to use prefix-LRE and so CCW fast-trans and MDC were inadvertently lost in those environments.

Undocumented SSI Monitor Bits: In z/VM 6.2 D1 R1 MRMTREPR and D1 R9 MRMTRSPR were given bits to indicate that the SSI domain is active for sample and event data. IBM inadvertently omitted documentation of these bits on's compendium of z/VM 6.2 monitor records. The web site has been repaired.

Missing RDEV Fields: D6 R7 MRIODENB, D6 R8 MRIODDSB, and D6 R31 MRIODMDE were updated to include the RDEV number. This will help authors of reduction programs.

VMDSTATE Comments: In D2 R25 MRSCLDDL, D4 R4 MRUSEINT, and D4 R10 MRUSEITE, the comments for the VMDSTATE bit were repaired to describe VMDSUSPN correctly.

Counts of CPUs: D0 R16 MRSYTCUP and D0 R17 MRSYTCUM were changed to include a count of the number of CPUs described. Further, a continuation scheme was introduced so that if the number of CPUs needing to be described will not fit in a single record, multiple records per interval will be emitted and said records will indicate the chaining.

Missing base-VCPU bit: D4 R2 MRUSELOF was improved so that its CALFLAG1 field contains the CALBASE bit. This lets the reduction program discern which of the MRUSELOF records is for the base VCPU.

Nomenclature for High Performance FICON: MRIODDEV, MRSYTSYG, and MRSYTEPM were improved to use the term zHPF to refer to High Performance FICON.

Spelling errors: D6 R3 MRIODDEV, D6 R19 MRIODTOF, D6 R18 MRIODTON, and D1 R6 MRMTRDEV all were updated to repair spelling problems.

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