IBM 000-8697 Instrukcja Użytkownika Pobierz pdf (Strona 137)

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System Architecture 2-43

Managing Shared-Memory Resources

Semaphores

When an OnLine user process attempts to acquire a latch and ﬁnds that the

latchisunavailable,theuserprocesscanblockuntilthelatchisavailable.The

mechanism that signals the process to wake when the latch becomes

available is a UNIX semaphore.

The semaphore mechanism works likes this. Every OnLine user process is

associated with a semaphore. If a user process ﬁnds a latch unavailable, the

semaphore associated with the process is placed on a list of waiting

semaphores.Whentheuserprocessholdingthelatchisreadytoreleaseit,the

holding user process looks to see if any user processes are waiting for the

latch. If so, the holding process releases the latch and wakes the ﬁrst appro-

priate user process in the semaphore list.

Ifthe latch tobereleasedisa bufferlatch, the holdinguserprocesswakesthe

ﬁrst waiting process that has a compatible lock access type. (Refer to

page 2-60 for further information about buffer acquisition.)

All semaphores are created when shared memory is created. In most UNIX

operating systems, the number of semaphores permitted is equal to the

maximum number of concurrent user processes, speciﬁed as USERS in the

OnLine conﬁguration ﬁle.

UNIXkernelparameterscanaffectthenumberofsemaphorescreatedbyyour

UNIX operating system. (Refer to page 2-18 for a description of the role

played by UNIX kernel parameters.)

Forced Abort

If you explicitly kill a user process that is holding a latch, OnLine immedi-

atelyinitiatesanaborttopreservedataconsistency.IfanOnLineuserprocess

is holding a latch, the implication is that the user process is intent on

modifying shared memory. When a user process terminates, the tbinit

daemoninitiates propercleanup, releasingall locks andotherresourcesheld

by the user process.

Although tbundo can perform routine cleanup for processes that die prema-

turely, data consistency prevents tbinit from releasing shared-memory

latchesaspartofcleanup.Itisimpossible for tbinit todeterminewhetherthe

user process concluded its modiﬁcations before it was terminated or if the

database is in a consistent state.

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