Performance Considerations

As customers begin to deploy z/VM 7.1, they might wish to give consideration to the following items.

Increased Thread Density

During development of z/VM 7.1 IBM discovered the dispatcher was failing to wake up a logical processor in a certain situation related to SMT-2. Consider two sibling logical processors, A and B, of a core. When B was in a wait state and A queued work onto the DSVBK they share, A failed to wake up B to help with the work. z/VM 7.1 repairs this. Some customers might see increased thread density, which is usually indicative of better exploitation of the core. However, because thread density can affect logical processor throughput, said customers might also see reduced single-processor throughput. Refer to the SMT article for a discussion of the considerations around exploiting SMT-2.

Decreased Dump Time

The dump enhancements reduce required dump time and required dump space, usually without compromising the utility of the dump. Customers might wish to review local startup procedures to ensure CP SET DUMP PGMBKS NONE FRMTBL NO gets issued at startup. Customers might also wish to revise local operations procedures to assure that when CP SNAPDUMP is issued, the PGMBKS NONE and FRMTBL NO operands get used. Finally, customers should remain aware that the stand-alone dump utility always dumps PGMBKs and always dumps the frame table.