Single-processor systems

The SINGLE_CPU_VP parameter should set to 1, this avoids the overhead of having to lock data structures to prevent other CPUs from using the same data space.

Multi-Processor

For two processor system the performance it is sometimes better leaving SINGLE_CPU_VP parameter at 1 as the overhead of maintaining the mutex outweighs the benefits of running two CPU vps. This is less likely with the latest high speed processors. The larger the number of physical processors the greater the potentail gain from running mulitple CPU vps.

Only add one CPU vp at a time, and monitor it's effectiveness before deciding to add another.

In general do not add more CPU vps than processors, NUMCPUVPS = physical CPUS - 1. But as already mentioned the latest processors have started to change this to NUMCPUVPS <= 2 * Number of physical processors

If the processors are already running at or near 0% idle then adding another CPU vp will probably degrade performance.

With multi-processors boxes it is possible to bind CPU vps to physical processors. Sometimes this binding has to be done at an OS level and not an Informix level, this will be in the release notes. Always leave at least one processor free for other vps and processes to run on. Multiprocessors systems support the following configuration parameters.

AFF_NPROCS	The number of processors to assign CPU vps to processors serially. If this value is zero, the CPU vp will be assigned to a specific processor.
AFF_SPROC	The processor (starting at zero) that OnLine will start with in assigning CPU vps to processors.

If AFF_NPROCS + APP_SPROC is greater than the number of hardware processors, an error will be displayed and affinity will not be used.