Kernel Programming Guide: Overview of Scheduling

The Mac OS X scheduler is derived from the scheduler used in OSFMK 7.3. In general, much documentation about prior implementations applies to the scheduler in Mac OS X, although you will find numerous differences. The details of those differences are beyond the scope of this overview.

Mach scheduling is based on a system of run queues at various priorities that are handled in different ways. The priority levels are divided into four bands according to their characteristics, as described in Table 9-1.

**Table 9-1** Thread priority bands
Priority Band	Characteristics
Normal	normal application thread priorities
System high priority	threads whose priority has been raised above normal threads
Kernel mode only	reserved for threads created inside the kernel that need to run at a higher priority than all user space threads (I/O Kit workloops, for example)
Real-time threads	threads whose priority is based on getting a well-defined fraction of total clock cycles, regardless of other activity (in an audio player application, for example).

Threads can migrate between priority levels for a number of reasons, largely as an artifact of the time sharing algorithm used. However, this migration is within a given band.

Threads marked as being real-time priority are also special in the eyes of the scheduler. A real-time thread tells the scheduler that it needs to run for A cycles out of the next B cycles. For example, it might need to run for 3000 out of the next 7000 clock cycles in order to keep up. It also tells the scheduler whether those cycles must be contiguous. Using long contiguous quanta is generally frowned upon but is occasionally necessary for specialized real-time applications.

The kernel will make every effort to honor the request, but since this is soft real-time, it cannot be guaranteed. In particular, if the real-time thread requests something relatively reasonable, its priority will remain in the real-time band, but if it lies blatantly about its requirements and behaves in a compute-bound fashion, it may be demoted to the priority of a normal thread.

In addition to the raw Mach RPC interfaces, some aspects of a thread’s priority can be controlled from user space using the POSIX thread priority API. The POSIX thread API is able to set thread priority only within the lowest priority band (0–63). For more information on the POSIX thread priority API, see “Using the pthreads API to Influence Scheduling”.




	Get information on Apple products. Visit the Apple Store online or at retail locations. 1-800-MY-APPLE Copyright © 2007 Apple Inc. All rights reserved. \| Terms of use \| Privacy Notice