线程内线程的优先级

Question

Suppose you have a program that starts two threads a and b , and b starts another ten threads of its own. Does a receive half of the available "attention" while b and its threads share the other half, or do they all share equally? If the answer is the latter by default, how could you achieve the former? Thanks!

Answer 1

There are lots of nice documentation on this topic. One such is this .

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:

1. A higher priority thread becomes "Runnable".
2. It yields, or its run() method exits.
3. On systems that support time-slicing, its time allotment has expired.

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.

Answer 2

Does a receive half of the available "attention" while b and its threads share the other half, or do they all share equally?

Neither. The proportion of time received by each thread is unspecified, and there's no reliable way to control it in Java. It is up to the native thread scheduler.

If the answer is the latter by default, how could you achieve the former?

You can't, reliably.

The only thing that you have to influence the relative amounts of time each thread gets to run are thread priorities. Even they are not reliable or predictable. The javadocs simply say that a high priority thread is executed "in preference to" a lower priority thread. In practice, it depends on how the native thread scheduler handles priorities.

For more details: http://docs.oracle.com/javase/7/docs/technotes/guides/vm/thread-priorities.html ... which includes information on how thread priorities on a range of platforms and Java versions.

Answer 3

One cannot say with surity the order in which the threads will be executed. Thread Scheduler works as per its inbuilt algorithm which we cannot change. Thread Scheduler picks up any threads (Highest priority threads) from runnable pool and make it running. We can only mention the priority in which scheduler should process our threads.