Red Hat APPLICATION STACK 1.1 RELEASE Manuale Utente Scaricare il pdf (Pagina 14)

Chapter 1: Software Overview

Fast

gettimeofday

Library Function

In order to correctly order operations in a database, the database must timestamp data

and log entries frequently. Traditional gettimeofday(3)library functions are implemented

as system calls, thus causing an address space switch (from user to kernel mode) each

time a timestamp is required.

With this release, SGIhas includeda device driverthat allowsa read-onlypage of kernel

memory,containing only the time value, to be mapped by an application (or on its behalf

by alibrary function).gettimeofday is then implemented as a library functionthat simply

reads the time value from the exported page in memory, thus avoiding a system call to

obtain the time value.

In order to use the Fast gettimeofday(3) Library Function, the kernel must be compiled

with CONFIG_SYSDAT and the user program must link against libdba in order to

override the libc version of gettimeofday(3).

POSIX Asynchronous I/O

The ability to overlap I/O and processing activities has always been important to

high-performance applications. To allow this type of overlap in single-threaded

applications, SGI has included a kernel-level implementation of POSIX asynchronous

I/O and the associated API library.

This facility is turned on by setting the CONFIG_AIO kernel option. User code can get

access to the facility by linking with libdba.so. Further information can be found in the

lib/libdba/README ﬁle.

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Red Hat APPLICATION STACK 1.1 RELEASE Manuale Utente Pagina 14