AIX Version 4.3 Differences Guide

4.3.2 The M:N Model
In the M:N model, there are two underlying types of pthreads. Those with
PTHREAD_SCOPE_SYSTEM, or system scope contention, otherwise known as
global threads, and those with PTHREAD_SCOPE_PROCESS, or process scope
contention. These threads are known as local threads. There are also two types
of thread schedulers in the system. The AIX kernel scheduler schedules all kernel
threads. There is also a user thread scheduler, which schedules the local
pthreads in a process.
Global threads are mapped 1:1 to kernel threads, and hence, are scheduled
exclusively by the AIX kernel scheduler. The 1:1 threads model used by prior
releases of AIX Version 4 only uses global threads.
Process A
Process B
G L L L L L L
User
Pthreads
User Scheduler
VP VP VP
User Scheduler
VP
Pthreads
Library
K
K
K
K
Kernel
Threads
Kernel Scheduler
Kernel
Scheduler
CPU
CPU
Figure 8. M:N Threads Model
The local pthreads are multiplexed over a set of kernel threads by the user
scheduler, which is part of the pthreads library.
4.3.3 User Scheduler
The user scheduler is run on a dedicated hidden pthread, which is created when
the pthreads library is initialized in M:N mode at process startup. There is one
user scheduler for each process using the M:N model. The hidden pthread is
created with system scope contention and therefore is scheduled directly by the
kernel scheduler.
72 AIX Version 4.3 Differences Guide