The following items improve performance:
- Contiguous Frame Management Improvements
- Improved Management of Idle Guests with Pending Network I/O
- No Pageable CP Modules
The use of the 64-bit architecture by CP results in a greater demand for data structures that require contiguous frames, particularly those associated with dynamic address translation. This line item involves various algorithmic changes to improve the management of contiguous frames. These changes have the potential to improve system performance and avoid potential system hangs when memory is constrained and/or fragmented. While very few field problems have been identified in existing releases, these improvements help ensure that systems will continue to run well as memory usage increases.
A problem was found on previous VM releases where guests were not being dropped from the dispatch list when they went idle. This was because those guests had network I/O outstanding even though network I/O is often long-term. Such guests appeared runnable with high I/O active wait state percentages.
APAR VM63282 addresses this problem for fully simulated network devices. 1 Guests that have outstanding network I/O to such devices but are otherwise idle are now considered to be idle. This causes applicable guests to be appropriately dropped from the dispatch list, allowing their storage to be more effectively identified as available for other purposes. It also causes their user state sampling to shift from I/O active wait state to idle or test idle state. This APAR has been integrated into z/VM 5.1.0. The integrated version has been extended so that it also applies to real devices that are attached to a virtual machine and that have the same device codes as those supported by the APAR.
With z/VM 5.1.0, all CP modules now reside in fixed storage. Measurement results indicate that this has resulted in a small net performance improvement in most situations due to reduced module linkage overhead. Years ago, when real storage sizes were much smaller, the ability to make infrequently used CP modules pageable provided a meaningful performance advantage but now storage sizes are so large that this design is no longer necessary.
VM63282 is available for z/VM 4.3.0 (PTF UM30888) and z/VM 4.4.0 (PTF UM30889). It applies to virtual (simulated) devices with the following properties:
VDEVCLAS=CLASSPEC VDEVTYPE=TYPOSA OSA2 / OSA-E QDIO / HiperSockets VDEVCLAS=CLASSPEC VDEVTYPE=TYPFCP FCP subchannel VDEVCLAS=CLASSPEC VDEVTYPE=TYPCTCA CTCA / 3088 / ESCON / FICON VDEVCLAS=CLASSVCM VDEVTYPE=TYPMSGF Msg Facility device
Virtual devices are those created by CP DEFINE commands, SPECIAL directory statements and NICDEF directory statements.