Threads - Thread Scheduling

• execution of multiple threads in some order on a single CPU system is called scheduling
• Java uses fixed-priority scheduling algorithms to decide which thread to execute
• the thread with the highest priority runs first
• if another thread with a higher priority is started, Java makes the lower priority thread wait
• if more than one thread exists with the same priority, Java quickly switches between them in round-robin fashion BUT only if the operating system uses time-slicing (see below)

Priorities

• it's possible to assign a thread priority
• the Thread class contains three integer priority constants
  1. [ 1] MIN_PRIORITY
  2. [ 5] NORM_PRIORITY
  3. [10] MAX_PRIORITY

• the default thread priority is NORM_PRIORITY
• when a thread is created, it takes the priority of the thread which created it
• you can check a threads priority using getPriority()
• you can change a threads priority using setPriority()
• if you change the priority on an executing thread to a lesser priority, it may stop executing as there may be another thread with a higher-priority (BB pg 259)

Actual Scheduling depends on the OS

• the above act as a guide to scheduling however the actual implementation depends on the Operating System
• most operating systems use one of two scheduling methods
  1. Preemptive scheduling
  2. Time slicing
• In preemptive scheduling the highest priority thread continues to run until it dies, waits, or is preempted by a thread of higher priority
• In time slicing a thread runs for a specific time and then enters the runnable state; at which point the scheduler decides wether to return to the thread or schedule a different thread (method used by Win95/NT)

• DO NOT rely on thread priority as a guarantee that the highest priority thread will always be running; the operating system has the final say
• priorities are used as guides to efficiency
• priority manipulations CANNOT be used as a substitute for locking (see synchronization)

General Conventions for setting priorities (CPJ pg 16)

• following represent the general conventions for setting thread priorities based on the type of activity the thread is involved in

    Range              Use
    -----   ----------------------------
      10    Crisis management
     7-9    Interactive, event-driven
     4-6    IO
     2-3    Background computation
      1     Run only if nothing else can