The database server virtual processors attach to shared memory during setup. During this process, the database server must satisfy the following two requirements:
The database server uses two configuration parameters, SERVERNUM and SHMBASE, to meet these requirements.
When a virtual processor attaches to shared memory, it performs the following major steps:
The operating system returns the shared-memory identifier for the first shared-memory segment.
The following sections describe how the database server uses the values of the SERVERNUM and SHMBASE configuration parameters in the process of attaching shared-memory segments.
The values of the SERVERNUM configuration parameter and shmkey, an internally calculated number, determine the unique key value for each shared-memory segment.
To see the key values for shared-memory segments, run the onstat -g seg command. For more information, see the sections on SHMADD and the buffer pool in your IBM Informix Dynamic Server Performance Guide.
When a virtual processor requests that the operating system attach the first shared-memory segment, it supplies the unique key value to identify the segment. In return, the operating system passes back a shared-memory segment identifier associated with the key value. Using this identifier, the virtual processor requests that the operating system attach the segment of shared memory to the virtual-processor address space.
The SHMBASE parameter in the ONCONFIG file specifies the virtual address where each virtual processor attaches the first, or base, shared-memory segment. Each virtual processor attaches to the first shared-memory segment at the same virtual address. This situation enables all virtual processors within the same database server instance to reference the same locations in shared memory without needing to calculate shared-memory addresses. All shared-memory addresses for an instance of the database server are relative to SHMBASE.
The value of SHMBASE is sensitive for the following reasons:
For information on SHMBASE, see your Dynamic Server machine notes.
Each virtual processor must attach to the total amount of shared memory that the database server has acquired. After a virtual processor attaches each shared-memory segment, it calculates how much shared memory it has attached and how much remains. The database server facilitates this process by writing a shared-memory header to the first shared-memory segment. Sixteen bytes into the header, a virtual processor can obtain the following data:
To attach additional shared-memory segments, a virtual processor requests them from the operating system in much the same way that it requested the first segment. For the additional segments, however, the virtual processor adds 1 to the previous value of shmkey. The virtual processor directs the operating system to attach the segment at the address that results from the following calculation:
SHMBASE + (seg_size x number of attached segments)
The virtual processor repeats this process until it has acquired the total amount of shared memory.
Given the initial key value of (SERVERNUM * 65536) + shmkey, the database server can request up to 65,536 shared-memory segments before it could request a shared-memory key value used by another database server instance on the same computer.
If your operating system uses a parameter to define the lower boundary address for shared memory, and the parameter is set incorrectly, it can prevent the shared-memory segments from being attached contiguously.
Figure 27 illustrates the problem. If the lower-boundary address is less than the ending address of the previous segment plus the size of the current segment, the operating system attaches the current segment at a point beyond the end of the previous segment. This action creates a gap between the two segments. Because shared memory must be attached to a virtual processor so that it looks like contiguous memory, this gap creates problems. The database server receives errors when this situation occurs.
To correct the problem, check the operating-system kernel parameter that specifies the lower-boundary address or reconfigure the kernel to allow larger shared-memory segments. For a description of the operating-system kernel parameter, refer to Shared-Memory Lower-Boundary Address (Windows).