flyinweb's blog

Until Informix Dynamic Server 10.0 the number of MSC (or Miscellaneous) Virtual Processors was limited to one. With IDS 10.0 multiple MSC VPs can be configured using the VPCLASS onconfig parameter (e.g. VPCLASS msc,num=2), or added dynamically using onmode -p 1 msc.

What does that mean and why would I want to use it?
The MSC VP is a type of AIO VP, executing operating system calls on behalf of other threads. While the AIO VP mainly takes care of filesystem I/O calls the MSC VP executes some network and authentication calls like getpwnam(), gethostbyname(), gethostbyaddr() etc. (It also maintains some statistics like VP CPU usage.) The MSC VP gets used at client connection time for the operating system part of user validation, and in most cases one is sufficient.

Virtual Processor (VP) is the technical term for an Informix process. Informix forks off a handful of these VPs at engine startup to take advantage of multiple CPUs or cores and increase parallelism. Each VP is multithreaded allowing Informix to efficiently use the CPU time each process is awarded by the OS, letting ready threads do work while other threads are busy waiting on something else to finish.

There are multiple types of VPs, each performing a specific job. Some VPs are configurable (i.e. the number of VPs and what CPU they should run on, etc.) through the VPCLASS ONCONFIG parameter and some are not. The 2 VPs you need to be concerned about right now are:

• CPU - Runs all session threads and some system threads. Runs KAIO (Kernel Asynchronous I/O) if enabled. This process will consume most of the CPU cycles given to Informix by the OS and is configurable.
• AIO - Performs all I/O except for physical or logical logging if KAIO is disabled. Performs I/O to cooked chunks if KAIO is enabled and is configurable.