Programming Interviews Exposed: Secrets to Landing Your Next Job, 3rd Edition

Concurrency Problems

Issues that you encounter with threads in professional development can be Byzantine in their complexity, but concise thread problems appropriate for an interview are difficult to compose. Therefore the questions you get are likely to come from a fairly small set of classic thread problems, several of which are presented here.

Busy Waiting

PROBLEM Explain the term “busy waiting” and how it can be avoided.

This is a simple problem, but one with important performance implications for any multithreaded application.

Consider a thread that spawns another thread to complete a task. Assume that the first thread needs to wait for the second thread to finish its work, and that the second thread terminates as soon as its work is done. The simplest approach is to have the first thread keep checking whether the second thread is alive and proceed as soon as it is dead:

Thread task = new TheTask();
task.start();
while( task.isAlive() ){
    ; // do nothing
}

This is called busy waiting because the waiting thread is still active, but it’s not actually accomplishing anything. It’s “busy” in the sense that the thread is still executed by the processor, even though the thread is doing nothing but waiting for the second thread to finish. Typically there are more active threads than cores, so this actually “steals” processor cycles away from the second thread (and any other active threads in the system), cycles that could be better spent doing real work.

Busy waiting is avoided ...