Trail:
Essential Java Classes
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Lesson:
Doing Two or More Tasks At Once: Threads
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Understanding Thread Priority
Previously, this lesson claimed that threads run concurrently.
While conceptually this is true, in practice it usually isn't. Most computer
configurations have a single CPU, so threads actually run one at a time
in such a way as to provide an illusion of concurrency. Execution of multiple
threads on a single CPU, in some order, is called scheduling.
The Java runtime supports a very simple, deterministic scheduling
algorithm known as fixed priority scheduling. This algorithm
schedules threads based on their priority relative to
other runnable threads.
When a Java thread is created, it inherits its priority from the
thread that created it. You can also modify a thread's priority at
any time after its creation using the setPriority method.
Thread priorities are integers ranging between
MIN_PRIORITY and MAX_PRIORITY (constants defined in the Thread class).
The higher the integer, the higher the priority.
At any given time, when multiple threads are ready to be executed,
the runtime system chooses the runnable thread with the highest
priority for execution. Only when that thread stops, yields, or becomes
not runnable for some reason
will a lower priority thread start executing.
If two threads of the same priority are waiting for the CPU,
the scheduler chooses one of them to run in a round-robin fashion.
The chosen thread will run until one of the following conditions is true:
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A higher priority thread becomes runnable.
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It yields, or its
run method exits.
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On systems that support time-slicing, its time allotment has expired.
Then the second thread is given a chance to run, and so on, until the
interpreter exits.
The Java runtime system's thread scheduling algorithm is also preemptive. If at
any time a thread with a higher priority than all other runnable threads
becomes runnable, the runtime system chooses the new higher priority
thread for execution. The new higher priority thread is said to preempt
the other threads.
Rule of thumb: At any given time, the highest priority
thread is running. However, this is not guaranteed. The thread scheduler
may choose to run a lower priority thread to avoid starvation. For this
reason, use priority only to affect scheduling policy for efficiency
purposes. Do not rely on thread priority for algorithm correctness.
The 400,000 Micron Thread Race
This Java source code implements
an applet that animates a race between two "runner" threads with
different priorities. When you click the mouse down over the applet,
it starts the two runners. The top runner, labelled "2", has a priority
of 2. The second runner,
labelled "3", has a priority of 3.
Try this: Click the applet below to start the race.
Note: Because some old browsers don't support 1.1, the above applet is a 1.0 version (here is the 1.0 code; here's the 1.1 code). To run the 1.1 version of the applet, go to example-1dot1/RaceApplet.html. For more information about running applets, refer to About Our Examples.
This is the run method for both
runners.
public int tick = 1;
public void run() {
while (tick < 400000)
tick++;
}
This run method simply counts from 1 to 400,000.
The instance variable tick is public because the
applet uses this value to determine how far the runner has progressed
(how long its line is).
In addition to the two runner threads, this applet also has a third
thread that handles the drawing. The drawing thread's run
method contains an infinite loop; during each iteration of the loop it
draws a line for each runner (whose length is computed from the runner's
tick variable), and then sleeps for 10 milliseconds.
The drawing thread has a thread priority of 4--higher than either runner.
So, whenever the drawing thread wakes up after 10 milliseconds, it
becomes the highest priority thread, preempting whichever runner is
currently running, and draws the lines. You can see the lines
inch their way across the page
This is not a fair race because one runner has
a higher priority than the other. Each time the drawing thread
yields the CPU by going to sleep for 10 milliseconds, the scheduler
chooses the highest priority runnable thread to run; in this case,
it's always runner 3. Here is another version
of the applet that implements a fair race, that is, both of the
runners have the same priority and they have an equal chance of
being chosen to run.
Try this: Click the mouse to start the race.
Note: Because some old browsers don't support 1.1, the above applet is a 1.0 version (here is the 1.0 code; here's the 1.1 code). To run the 1.1 version of the applet, go to example-1dot1/RaceApplet.html. For more information about running applets, refer to About Our Examples.
In this race, each time the drawing thread yields the CPU by
going to sleep, there are two runnable threads of equal
priority--the runners--waiting for the CPU; the scheduler
must choose one of the threads to run. In this situation,
the scheduler chooses the next thread to run in a round-robin
fashion.
Selfish Threads
The Runner class used in the races above actually implements "socially-impaired"
thread behavior. Recall the run method from the Runner class used
in the races above:
public int tick = 1;
public void run() {
while (tick < 400000)
tick++;
}
The while loop in the run method is in a tight loop.
Once the scheduler chooses a thread with this thread body
for execution, the thread never voluntarily relinquishes control of the CPU--the
thread continues to run until the while loop terminates naturally
or until the thread is preempted by a higher priority thread.
This thread is called a selfish thread.
In some situations, having selfish threads doesn't cause any problems because
a higher priority thread preempts the selfish one (just as the drawing thread
in the RaceApplet preempts the selfish runners). However, in other situations,
threads with CPU-greedy run methods, such as the Runner class,
can take over the CPU and cause other threads to wait for a long
time before getting a chance to run.
Time-Slicing
Some systems, such as Windows 95/NT, fight selfish thread behavior with a strategy known as
time-slicing. Time-slicing comes into play when there are multiple
"Runnable" threads of equal priority and those threads are the highest priority
threads competing for the CPU. For example, this
stand-alone Java program (which is based on
the RaceApplet above) creates two equal priority
selfish threads
that have the following run method.
public void run() {
while (tick < 400000) {
tick++;
if ((tick % 50000) == 0)
System.out.println("Thread #" + num + ", tick = " + tick);
}
}
This run contains a tight loop that increments the integer tick
and every 50,000 ticks prints out the thread's identifier and its tick count.
When running this program on a time-sliced system, you will see messages from both
threads intermingled with one another. Like this:
Thread #1, tick = 50000
Thread #0, tick = 50000
Thread #0, tick = 100000
Thread #1, tick = 100000
Thread #1, tick = 150000
Thread #1, tick = 200000
Thread #0, tick = 150000
Thread #0, tick = 200000
Thread #1, tick = 250000
Thread #0, tick = 250000
Thread #0, tick = 300000
Thread #1, tick = 300000
Thread #1, tick = 350000
Thread #0, tick = 350000
Thread #0, tick = 400000
Thread #1, tick = 400000
This output is produced because a time-sliced system divides the CPU into time slots and iteratively
gives each of the equal-and-highest priority threads a time slot in which to run.
The time-sliced system iterates through the equal-and-highest priority
threads, allowing each one a bit of time to run, until one or more of them finishes
or until a higher priority thread preempts them. Notice that time-slicing makes no guarantees as to
how often or in what order threads are scheduled to run.
When running this program on a non-time-sliced system, however, you will see messages
from one thread finish printing before the other thread ever gets a chance to print
one message. Like this:
Thread #0, tick = 50000
Thread #0, tick = 100000
Thread #0, tick = 150000
Thread #0, tick = 200000
Thread #0, tick = 250000
Thread #0, tick = 300000
Thread #0, tick = 350000
Thread #0, tick = 400000
Thread #1, tick = 50000
Thread #1, tick = 100000
Thread #1, tick = 150000
Thread #1, tick = 200000
Thread #1, tick = 250000
Thread #1, tick = 300000
Thread #1, tick = 350000
Thread #1, tick = 400000
This is because a non-time-sliced system chooses one of the equal-and-highest
priority threads to run and allows that thread to run until it relinquishes the
CPU (by sleeping, yielding, finishing its job) or until a higher priority preempts it.
Note: The Java runtime does not implement (and therefore does
not guarantee) time-slicing. However, some systems on which you can run Java
do support time-slicing. Your Java programs should not rely on time-slicing as it
may produce different results on different systems.
Try this: Compile and run the RaceTest
and SelfishRunner classes on
your computer. Can you tell if you have a time-sliced system?
As you can imagine, writing CPU-intensive code can have negative repercussions
on other threads running in the same process. In general, you should try to
write "well-behaved" threads that voluntarily relinquish the CPU periodically and
give other threads an opportunity to run. In particular, you should never
write Java code that relies on time-sharing--this will practically guarantee
that your program will give different results on different computer systems.
A thread can voluntarily yield the CPU without going to sleep or some other
drastic means by calling the yield method. The yield
method gives other threads of the same priority a chance to run. If there
are no equal priority threads that are runnable, then the yield is ignored.
Try this: Rewrite the SelfishRunner class to be a
PoliteRunner by calling the
yield method from the run method.
Be sure to modify the main program
to create PoliteRunners instead of SelfishRunners. Compile and run the new
classes on your computer. Now isn't that better?
Summary
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Most computers have only one CPU, so threads must share
the CPU with other threads. The execution of multiple threads on a
single CPU, in some order, is called scheduling. The Java runtime
supports a very simple, deterministic scheduling algorithm known as
fixed priority scheduling.
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Each Java thread is given a numeric priority between
MIN_PRIORITY and
MAX_PRIORITY (constants defined in the Thread class). At any given time,
when multiple threads are ready to be executed, the thread with the
highest priority is chosen for execution. Only when that thread
stops, or is suspended for some reason, will a lower priority thread
start executing.
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Scheduling of the CPU is fully preemptive. If a thread with a higher
priority than the currently executing thread needs to execute, the
higher priority thread is immediately scheduled.
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The Java runtime will not preempt the currently running thread for
another thread of the same priority. In other words, the Java runtime
does not time-slice. However, the system implementation of threads
underlying the Java Thread class may support time-slicing.
Do not write code that relies on time-slicing.
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In addition, a given thread may, at any time, give up its right to
execute by calling the
yield method. Threads can only
yield the CPU to other threads of the same priority--attempts to
yield to a lower priority thread are ignored.
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When all the runnable threads in the system have the same priority,
the scheduler chooses the next thread to run in a simple, non-preemptive,
round-robin scheduling order.
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