Multiprocessors

Question: Do I Run my Two-processor System with MULTIPROCESSOR = 1 or 0?

I have a system with two physical processors. Do I set my system up with the MULTIPROCESSOR flag set to 0 for no multiprocessors or set to 1 for multiprocessors. How many CPU-VPs should I set up?

Answer: It depends.

This is a somewhat noncommittal answer. The only way to find out for sure is to test it with your application on your own server. "Conventional wisdom" for Informix says that the overhead of handling the processor switching code that is implemented when you run with MULTIPROCESSOR = 1 eclipses the performance gains with only two processors and that this flag should only be set when you have four or more processors. There has been some recent anecdotal evidence that shows that this may no longer be valid. Some users claim that this is valid with slower processors but not valid with processors over about 200 MHz. With the rapid increase in processor speed, especially in NT systems, this should be tested rather than taken for granted.

Question: How many CPU-VPs Should I set up in a Multiprocessor System?

The manual suggests running with a number of CPU-VPs one fewer than the number of physical processors. Is this still true?

Answer: It Depends upon your System and Application.

The manual recommendation is based on leaving one processor always available for non-database operating system activities, thus allocating #CPUs - 1 to the database. Again, with faster processors, this may not be accurate. With systems in which the only application running on the server is the database engine itself, it may pay to go ahead and increase the number of CPU-VPs up to the number of physical CPUs.

On decision support systems that make extensive use of PDQ and parallel index builds and which have multiple, fast processors, some users report up to a 60 percent improvement in index build times by increasing the number of CPU-VPs to one plus the number of processors. I've seen claims that having up to twice the number of CPU-VPs in relation to physical CPUs will increase the speed of the system.

You've got to remember that as systems increase in speed, they become more efficient and can handle such things as thread switching much faster than before. The manuals and even the conventional wisdom from the Internet discussion groups cannot always keep pace with the increase in capability of the technology. You've got to test these things for yourself on your own system.