IBM 000-8697 Bedienungsanleitung PDF herunterladen (Seite 120)

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2-26 IBM Informix OnLine Database Server Administrator’s Guide

How User Processes Attach to Shared Memory

The ﬁrst shared-memory segment is attached to the virtual address space of

each process at the same virtual address deﬁned as SHMBASE. SHMBASE

identiﬁes the speciﬁc virtual address where the database server processes

attach the ﬁrst, or base, shared-memory segment. (Refer to Figure 2-2 on

page 2-23 for an illustration of the virtual address space of a database server

process.)

The reason thatall user processessharethe sameSHMBASE value is to speed

execution. All user processes can reference locations in shared memory

withoutrecalculatingshared-memoryaddressesbecauseall addressesbegin

at the same base address. All addresses assume that shared memory begins

at the address speciﬁed as SHMBASE. That is, all addresses are relative to

SHMBASE. If each user process attached to shared memory at a different

location, shared-memory addresses would be relative to the start of shared

memory and would have to be recalculated for each user process, slowing

execution.

The speciﬁc value of SHMBASE is often machine-dependent. It is not an

arbitrary number. Informix selects a value for SHMBASE that will keep the

shared-memorysegments safe incase theuser process dynamically acquires

additional memory space.

DifferentUNIXsystemsaccommodateadditional memoryatdifferentvirtual

addresses.SomeUNIX architecturesextendthe highest virtualaddressof the

user process data segment to accommodate the next segment. In this case, it

is possible that the data segment could grow into the shared-memory

segment.

The server process function stack or (heap) can pose another threat. Some

UNIX architecturesbegin the function stack at a high virtual address tokeep

it clear of the growing data segments. If the stack begins too close to the

shared-memorysegments,thestackcanoverwritetheendofsharedmemory.

Some versions of UNIX require the user to specify a SHMBASE of virtual

address of 0. The 0 address informs the UNIX kernel that the kernel should

pick the best address at which to attach the shared-memory segments. This

kernel-selects option is an attemptto respond to the many differentways that

anapplicationcanaffectthegrowthof the serverprocess.However, allUNIX

architecturesdonotsupportthekernel-selectsoption. Moreover,thekernel’s

selection is not always the best choice for all applications.

Informix recommends that you do not attempt to change the value of

SHMBASE.

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