IBM 000-8697 Manual Do Utilizador

IBM Informix OnLine Database Server Administrator’s Guide

Each parallel sort uses one

UNIX

 shared-memory segment.

A front-end process is able to open an unlimited number of

SELECT

cursors

that contain an

ORDER BY

 clause. However, the number of sorts that can be

executed in parallel is limited by the number of shared-memory segments
that each OnLine database server process can attach to. (For more details
about the

UNIX

 parameter that specifies the maximum number of attached

shared-memory segments per process, refer to

.) All cursors beyond

the

UNIX

 limit are executed using nonparallel sorts.

Familiarize yourself with the function of a shared-memory latch before you
begin this section. Refer to

 for background information.

When an OnLine user process attempts to acquire a latch, it tests for avail-
ability. If the latch is not available, the user process can either wait or not wait.
A third option is available on some multiprocessor

UNIX

operating systems:

spin and test again.

In a multiprocessor environment, it is possible for two OnLine user processes
to be resident in a

CPU

and for both user processes to need access to the same

resource. Normally, one user process acquires the resource while the other
process waits. The waiting user process “sleeps,” meaning that the user
process is switched out of the

CPU

.

If a machine supports spin and test, the waiting user process does not sleep.
Instead, it “spins.” The “spinning” that the user process performs while it
waits is an assembly-level activity that varies among machines. The activity
itself does nothing.

The advantage of spinning and testing is that the waiting user process
remains in the

CPU

. This eliminates the overhead of context switching; that

is, the overhead that is incurred when the user process is switched in and out
of the

CPU

. The spin-and-test approach is more efficient than sleeping.