CPU Virtual Processors

The CPU virtual processor runs all session threads (the threads that process requests from SQL client applications) and some internal threads. Internal threads perform services that are internal to the database server. For example, a thread that listens for connection requests from client applications is an internal thread.

Determining the Number of CPU Virtual Processors Needed

The right number of CPU virtual processors is the number at which they are all kept busy but not so busy that they cannot keep pace with incoming requests. You should not allocate more CPU virtual processors than the number of hardware processors in the computer.

To evaluate the performance of the CPU virtual processors while the database server is running, repeat the following command at regular intervals over a set period of time:

If the accumulated usercpu and syscpu times, taken together, approach 100 percent of the actual elapsed time for the period of the test, add another CPU virtual processor if you have a CPU available to run it.

You should use the VPCLASS parameter instead of the AFF_SPROC, AFFNPROCS, NOAGE, NUMCPUVPS, and NUMAIOVPS parameters. The VPCLASS parameter cannot be used in combination with these parameters. If the ONCONFIG file contains a NUMCPUVPS parameter, for example, you receive an error message if the file also contains a VPCLASS cpu parameter. For information about the VPCLASS configuration parameter, refer to the IBM Informix Administrator's Reference.

In addition to considering the number of CPUs in the computer and the number of users who will connect to the database server, also consider the fact that user-defined routines and DataBlade modules, which are collections of user-defined routines, run on either CPU virtual processors or user-defined virtual processors. For information on how you assign a user-defined routine to a virtual processor, refer to Assigning a UDR to a User-Defined Virtual-Processor Class.

Running on a Multiprocessor Computer

If you are running multiple CPU virtual processors on a multiprocessor computer, set the MULTIPROCESSOR parameter in the ONCONFIG file to 1. When you set MULTIPROCESSOR to 1, the database server performs locking in a manner that is appropriate for a multiprocessor computer. For information on setting multiprocessor mode, refer to the chapter on configuration parameters in the IBM Informix Dynamic Server Administrator's Reference.

Running on a Single-Processor Computer

If you are running the database server on a single-processor computer, set the MULTIPROCESSOR configuration parameter to 0. To run the database server with only one CPU virtual processor, set the SINGLE_CPU_VP parameter to 1.

Setting MULTIPROCESSOR to 0 enables the database server to bypass the locking that is required for multiple processes on a multiprocessor computer. For information on the MULTIPROCESSOR configuration parameter, refer to the IBM Informix Dynamic Server Administrator's Reference.

Setting SINGLE_CPU_VP to 1 allows the database server to bypass some of the mutex calls that it normally makes when it runs multiple CPU virtual processors. For information on setting the SINGLE_CPU_VP parameter, refer to the IBM Informix Dynamic Server Administrator's Reference.

Setting VPCLASS num to 1 and SINGLE_CPU_VP to 0 does not reduce the number of mutex calls, even though the database server starts only one CPU virtual processor. You must set SINGLE_CPU_VP to 1 to reduce the amount of latching that is performed when you run a single CPU virtual processor.

Setting the SINGLE_CPU_VP parameter to 1 imposes two important restrictions on the database server, as follows:

Adding and Dropping CPU Virtual Processors in Online Mode

Preventing Priority Aging

Some operating systems lower the priority of long-running processes as they accumulate processing time. This feature of the operating system is called priority aging. Priority aging can cause the performance of database server processes to decline over time. In some cases, however, the operating system allows you to disable this feature and keep long-running processes running at a high priority.

To determine if priority aging is available on your computer, check the machine notes file, that comes with your installation and is described in the Introduction to this guide.

If your operating system allows you to disable priority aging, You can disable it by specifying noage for the priority entry in the VPCLASS configuration parameter. For more information, refer to the IBM Informix Dynamic Server Administrator's Reference.

Using Processor Affinity

The database server supports automatic binding of CPU virtual processors to processors on multiprocessor computers that support processor affinity. Your database server distribution includes a machine notes file that contains information on whether your database server version supports this feature. When you assign a CPU virtual processor to a specific CPU, the virtual processor runs only on that CPU, but other processes also can run on that CPU.

Use the VPCLASS configuration parameter with the aff option to implement processor affinity on multiprocessor computers that support it.

To see if processor affinity is supported on your UNIX platform, refer to the machine notes file.

Setting Processor Affinity with the VPCLASS Parameter

To set processor affinity with the VPCLASS parameter, specify the range of CPUs to which you are assigning the virtual processors. The database server assigns CPU virtual processors to CPUs in round-robin fashion, starting with the first processor number that aff specifies. If you specify more CPU virtual processors than there are processors, the database server starts over again at the beginning. In the following example, the affinity entry assigns a virtual processor to CPUs 1, 2, and 3. Three virtual processors (specified in num) are available:

In the following example, the database server assigns two virtual processors to CPUs 5 and 6, and one virtual processor to CPU 7:

For more information, see VPCLASS in the chapter on configuration parameters in the IBM Informix Dynamic Server Administrator's Reference.