Item 72: Don't depend on the thread scheduler

1.  The best way to write a robust, responsive, portable program is to ensure that the average number of runnable threads is not significantly greater than the number of processors. This leaves the thread scheduler with little choice: it simply runs the runnable threads till they’re no longer runnable. The program’s behavior doesn’t vary too much, even under radically different thread-scheduling policies.

 

2.  The main technique for keeping the number of runnable threads down is to have each thread do some useful work and then wait for more. Threads should not run if they aren’t doing useful work. In terms of the Executor Framework, this means sizing your thread pools appropriately, and keeping tasks reasonably small and independent of one another. Tasks shouldn’t be too small, or dispatching overhead will harm performance.

 

3.  Threads should not busy-wait, repeatedly checking a shared object waiting for something to happen. Besides making the program vulnerable to the vagaries of the scheduler, busy-waiting greatly increases the load on the processor, reducing the amount of useful work that others can accomplish.

 

4.  When faced with a program that barely works because some threads aren’t getting enough CPU time relative to others, resist the temptation to “fix” the program by putting in calls to Thread.yield. Thread.yield has no testable semantics. A better course of action is to restructure the application to reduce the number of concurrently runnable threads. 

 

5.  Thread priorities are among the least portable features of the Java platform. It is not unreasonable to tune the responsiveness of an application by tweaking a few thread priorities, but it is rarely necessary and is not portable. It is unreasonable to solve a serious liveness problem by adjusting thread priorities.

 

6.  It is within specification for Thread.yield to do nothing at all, simply returning control to its caller. Some modern VMs actually do this. Therefore, you should use Thread.sleep(1) instead of Thread.yield for concurrency testing.