AIX Version 4.3 Differences Guide

sharing the VPs. When the mutex lock is freed, the user scheduler examines the
list of threads waiting for the mutex and activates one of them.
4.3.5 Tuning
The M:N pthreads implementation provides several environment variables that
can be used to affect application performance. If possible, the application
developer should provide a front end shell script to invoke the binary executables
in which the user may specify new values to override the system defaults. The
following environment variables can be set by end users and are examined at
process initialization time.
AIXTHREAD_SCOPE
This variable can be used to set the contention scope of pthreads
created using the default pthread attribute object. It is represented by
the following syntax:
AIXTHREAD_SCOPE=[P|S]
The value P indicates process scope, while a value of S indicates
system scope. If no value is specified, then the default pthread attribute
object will use process scope contention.
AIXTHREAD_MNRATIO
This variable allows the user to specify the ratio of pthreads to kernel
threads. It is examined when creating a pthread to determine if a kernel
thread should also be created to maintain the correct ratio. It is
represented with the following syntax:
AIXTHREAD_MNRATIO=p:k
where k is the number of kernel threads to use to handle p pthreads.
Any positive integer value may be specified for p and k. These values
are used in a formula that employs integer arithmetic, which can result
in the loss of some precision when big numbers are specified. If k is
greater than p, then the ratio is treated as 1:1. If no value is specified,
the default ratio depends on the default contention scope. If system
scope contention is the default, the ratio is 1:1. If process scope
contention is set as the default, the ratio is 8:1.
AIXTHREAD_SLPRATIO
This variable is used to determine the number of kernel threads used to
support local pthreads sleeping in the library code on a pthread event.
For example, attempting to obtain a mutex. It is represented by the
following syntax:
AIXTHREAD_SLPRATIO=k:p
where k is the number of kernel threads to reserve for every p sleeping
pthreads. Notice that the relative positions of the numbers indicating
kernel threads and user pthreads are reversed when compared with
AIXTHREAD_MNRATIO. Any positive integer value may be specified
for p and k. These values are used in a formula that employs integer
arithmetic, which can result in the loss of some precision when large
numbers are specified. If k is greater than p, then the ratio is treated as
1:1. If the variable is not set, then a ratio of 1:12 is used.
The reason for maintaining kernel threads for sleeping pthreads is that,
when the pthread event occurs, the pthread will immediately require a
kernel thread to run on. It is more efficient to use a kernel thread that is
74 AIX Version 4.3 Differences Guide