Apache::Session::Lock:UseraContributed PerlApache::Session::Lock::Semaphore(3)NAMEApache::Session::Lock::Semaphore - Provides mutual exclusion through
semaphores
SYNOPSIS
use Apache::Session::Lock::Semaphore;
my $locker = new Apache::Session::Lock::Semaphore;
die "no semaphores" unless $locker;
$locker->acquire_read_lock($ref);
$locker->acquire_write_lock($ref);
$locker->release_read_lock($ref);
$locker->release_write_lock($ref);
$locker->release_all_locks($ref);
DESCRIPTION
Apache::Session::Lock::semaphore fulfills the locking interface of
Apache::Session. Mutual exclusion is achieved through system
semaphores and the IPC::Semaphore module.
CONFIGURATION
The module must know how many semaphores to use, and what semaphore key
to use. The number of semaphores has an impact on performance. More
semaphores means less lock contention. You should use the maximum
number of semaphores that your platform will allow. On stock NetBSD,
OpenBSD, and Solaris systems, this is probably 16. On Linux 2.2, this
is 32. This module tries to guess the number based on your operating
system, but it is safer to configure it yourself.
To set the number of semaphores, you need to pass an argument in the
usual Apache::Session style. The name of the argument is NSems, and the
value is an integer power of 2. For example:
tie %s, 'Apache::Session::Blah', $id, {NSems => 16};
You may also need to configure the semaphore key that this package
uses. By default, it uses key 31818. You can change this using the
argument SemaphoreKey:
tie %s, 'Apache::Session::Blah', $id, {NSems => 16, SemaphoreKey => 42};
PROBLEMS
There are a few problems that people frequently encounter when using
this package.
If you get an invalid argument message, that usually means that the
system is unhappy with the number of semaphores that you requested.
Try decreasing the number of semaphores. The semaphore blocks that
this package creates are persistent until the system is rebooted, so if
you request 8 semaphores one time and 16 semaphores the next, it won't
work. Use the system commands ipcs and ipcrm to inspect and remove
unwanted semphore blocks.
Cygwin
IPC on Cygwin requires running cygserver. Without it, program will exit
with "Bad System call" message. It cannot be intercepted with eval.
Read /usr/share/doc/Cygwin/cygserver.README for more information.
Darwin/MacOS X
Darwin and MacOS X may not have semaphores, see
<http://sysnet.ucsd.edu/~bellardo/darwin/sysvsem.html>
*BSD
Error "No space left on device" means that maximum number of semaphores
is reached. See
<http://www.postgresql.org/docs/7.3/static/kernel-resources.html> for
more information.
AUTHOR
This module was written by Jeffrey William Baker <jwbaker@acm.org>.
SEE ALSO
Apache::Session
perl v5.18.22014-05-1Apache::Session::Lock::Semaphore(3)