DB2 Server for VSE & VM: Performance Tuning Handbook

VM Specifics

Fair Share Scheduling

VM was originally designed to support a large number of equally important virtual machines. To ensure that each user receives an equal allotment of its resources, VM's scheduler attempts to give each machine in the system a fair share of the processor's time.

However, if you are only using one or two database machines that use most of your system's resources, fair share scheduling may keep them from receiving the processor time they need. The database and user machines may receive approximately the same resources to perform their tasks. However because the database machines are performing work for many users they may need much more resource than the users. The database machines may become a bottleneck, because the user machines are spending more time waiting for them than for processor time.

Fortunately, there are several parameters that let you shift fair share scheduling in the database machine's favor. Use these carefully. It is easy to over adjust them and virtually lockout all other users in your system.

SET SHARE

You can use the SET SHARE command or the SHARE directory statement to control the percentage of system resources a virtual machine receives. These resources include processors, real storage, and paging I/O capability. A virtual machine receives a proportion of any scarce resource according to its share setting.

You can use this command to ensure that a database machine receives an absolute minimum share of system resources and that the remaining resources are divided among the rest of the user machines. (However, remember that by allocating an absolute share of system resources to a single machine, you will also limit its share to that amount.

If the database machine is not the only multiple user server on the system, give it a relative (instead of absolute share).

SET QUICKDSP

You can use either the SET QUICKDSP command or the QUICKDSP operand of the OPTION directory statement to designate virtual machines that will not wait in the eligible list when they have work to do. Instead, a virtual machine with a quick dispatch setting (QUICKDSP) is added to the dispatch list immediately without first waiting in the eligible list.

You should always use this command to ensure that a database machine never waits longer than absolutely necessary for another machine when it has work to do.

For more information on either QUICKDSP or SHARE, refer to the VM/ESA: Planning and Administration manual.