IBM 000-8697 Manual de usuario descargar pdf (Pagina 113)

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System Architecture 2-19

UNIX Kernel and Semaphore-Allocation Parameters

■ The shared-memory lower-boundary address

■ The maximum number of attached shared-memory segments per

process

■ The maximum amount of shared memory system-wide

The remaining three parameters are semaphore-allocation parameters:

■ The maximum number of semaphore identiﬁers

■ The maximum number of semaphores

■ The maximum number of semaphores per identiﬁer

When tbinit creates the required shared-memory segments, it attempts to

acquire as large a segment as possible. The ﬁrst segment size tbinit tries to

acquire is the size of shared memory, rounded up to the nearest multiple of

2 KB.

OnLinereceivesanerrorfromtheoperatingsystem if therequestedsegment

size isgreaterthan themaximum allowable size. If OnLine receives anerror,

tbinit divides the requested size by 2 and tries again. For most installations,

more than one segment is required because of a UNIX kernel limitation.

Attempts at acquisition continue until the largest segment size that is a

multiple of 2 KB can be created. Then tbinit creates as many additional

segments as are required to meet shared-memory requirements.

Shared-memory identiﬁers affect OnLine operation when a user process

attempts to attach to shared memory. For most operating systems, there are

no limits on the number of shared-memory identiﬁers that a particular user

process can create or attach to. Instead, user processes receive identiﬁers on

a “ﬁrst come, ﬁrst served” basis, up to the limit that is deﬁned for the

operating system as a whole.

You mightbe able to calculate the maximum amount of shared memory that

the operating system can potentially allocate by multiplying the number of

shared-memory identiﬁers by the maximum shared-memory segment size.

Checkthatthemaximumamountofmemorythatcanbeallocatedisequalto

the total addressable shared-memory size for a single operating-system

process. The following display expresses the concept another way:

Maximum amount of shared memory =

(Maximum number of attached shared-memory segments per process) x

(Maximum shared-memory segment size)

1 2 ... 108 109 110 111 112 113 114 115 116 117 118 ... 701 702

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