IBM 000-8697 Bedienungsanleitung PDF herunterladen (Seite 136)

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2-42 IBM Informix OnLine Database Server Administrator’s Guide

Managing Shared-Memory Resources

If an OnLine user process requires a speciﬁc latch, how does it determine if

the latch is available? The user process has two options:

■ Test for the latch; if unavailable, do not wait (that is, do not block).

■ Test for the latch; if unavailable, wait (that is, block).

Test-and-Set Institutions

Mostmachinesuseasingletest-and-setinstructionaspartofthetestthateach

user process performs in its attempt to acquire a shared-memory latch. Test-

and-set, or TAS, prevents confusion between two user processes in a

multiuser environment. Without TAS, a user process could be interrupted

betweenthetestofthelatch(toseeifitisavailable)andthesettingofthelatch

(to make it unavailable to other user processes). The interrupt could create a

situation in whichmorethan oneuser process received “exclusive” access to

a resource.

To see how this confusion could occur, consider the following scenario.

Server Process A needsto acquire latch 201.ProcessA performs atest for the

latch and receives a positive response; the latch is available. Then the

processing time period for Process A ends. Process B begins executing.

Process B also needs latch 201. Process B performs a test for the latch and

receivesapositiveresponsesincethelatchis still available. ProcessBsets the

latch and continues processing. Process B is interrupted (that is, its timeslice

expired) before it is ready to release the latch. When Process A continues

executing, it incorrectly assumes that it can claim latch 201. The test-and-set

instruction performs the latch test and sets the latch as a single, uninter-

ruptable event, eliminating confusion among processes.

Spin and Test Again

WhenanOnLine user processattempts to acquirea latch, it teststhelatchfor

availability. If the latch is not available, the user process can either block or

notblock.AthirdoptionisavailableonsomemultiprocessorUNIXoperating

systems: spin and test again. The beneﬁt of spinning instead of blocking is

that a user process can test for latch availability multiple times without the

overhead cost of putting the user process to sleep and later waking it. The

conﬁguration parameter SPINCNT speciﬁes the number of times that a user

processcan spinand testwithout actuallygoing tosleep. (Referto page 5-24

for information about tuning this parameter to improve performance.)

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