IBM 000-8697 Manual de usuario descargar pdf (Pagina 114)

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2-20 IBM Informix OnLine Database Server Administrator’s Guide

UNIX Kernel and Semaphore-Allocation Parameters

If this relationship does not hold, either one of two undesirable situations

could develop:

■ If the total amount of shared memory is less than the total

addressable size, you are able to address more shared memory for

the operating system than that which is available.

■ If the total amount of shared memory is greater than the total

addressable size, you can never address some amount of shared

memory that is available. That is, space that could potentially be

used as shared memory cannot be allocated for that use.

OnLine operation requires one UNIX semaphore for each user structure in

sharedmemory. Duringshared-memory creation, tbinit attemptsto allocate

semaphores in blocks of 25. If the maximum number of semaphores per

shared-memory identiﬁer is fewer than 25, tbinit receives an error when it

attempts to allocate the semaphores. The maximum number of semaphore

identiﬁersshouldbeclosetothemaximumnumberofOnLineusersonahost

machine, divided by 25.

When OnLine user processes attach to shared memory, each user process

speciﬁes the address at which to attach the ﬁrst segment. This address is the

OnLine parameter SHMBASE.

OnLine assumes that the next segment can be attached at the address of the

previous segment, plus the size of the shared-memory segment; that is,

contiguous to the ﬁrst segment. However, your UNIX system might set a

parameter that deﬁnes the lower-boundary address of shared memory for

attaching shared-memory segments.If so, the next shared-memory segment

attempts to attach at the address of the previous segment, plus the value of

the lower-boundary address. If the lower-boundary address is greater than

the size of the shared-memory segment, the next segment is attached to a

point beyond the end of the previous segment, creating a gap between the

two segments. Since shared memory must be attached to a user process so

that it looks like contiguous memory, this gap creates problems. If this

situationoccurs,OnLinereceiveserrorsduringtheattach.Anexampleofthis

situation is shown in Figure 2-1.

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