Thread Management

There are two major issues with Java Threads:

  1. Concurrency
  2. Control

Just as there are two similar issues with juggling balls.

  1. When the balls are in the air, the balls may try to occupy the same space at the same time. A concurrency issue.

  2. When launching and catching the balls one needs to tightly coordinate the throw/catch so that a hand is free when a ball needs catching. A control issue.

Failure to address both these issues means the endeavor will fail sooner or later.

Concurrency

The good folks at the JSR166 expert group are doing a superb job of dealing with this issue. The current java.util.concurrent package and the soon to be released fork-join package represent many years of effort in development. Hats off to these people.

Control

Java threads are most difficult to control. What if a thread gets stuck in a blocking method? What if something is wrong and the thread doesn't get CPU time? What if there is a bug? There are lots of 'what if'' situations. See for example the Shadow.

Tymeac is a superb implementation of a very simple concept:

Put requests in queues for processing by asynchronous threads.

In a nutshell, Tymeac comprises queues with each queue having its own pool of threads. However, Tymeac Queue Threads are not your traditional pool threads. Every Queue Thread has its own management structure. Each event in the life of a Queue Thread is timed. Although there is no way to stop a thread, Tymeac handles the run-a-way or blocking-forever thread problems as best as can be handled at the application level.

Interrupt
Thread "interrupt()" is a disaster. The original developers probably had a vision that programmers would want to interrupt an executing thread. But they never perfected that vision. What we have now are threads interrupting themselves as well as other threads sometimes with erroneous results.

Lets say you create thread A and you expect that thread to complete some work within a time limit.

  • You execute a timed wait for thread A.
  • Thread A does not complete within the time limit,
    • the time expires and
    • you regain control.
    • Your code continues with other work.
  • Then you have a second timed wait for another thread B.
  • If thread A then issues interrupt(), it interrupts the caller at the second wait.

This can easily happen with packages you buy and cannot alter, especially message-writer methods. It is common practice to make a message-writer a separate thread so it doesnt affect the main thread if it hangs or causes an exception.

If you have a multi-thread environment (i.e. thread C, D etc.), then using interrupt() only exacerbates the problem.

interrupt() is like getting kicked in the butt you know youve been kicked but you dont know who did it, when or why.

Tymeac does not use "interrupt()". All communication with Queue Threads is done through the thread's management structure.

NotifyAll
Both NotifyAll() and SignalAll() are shot gun methods. Having multiple threads waiting on a single object is a course grained solution. When the group awakens every thread must do some work to find out if it is needed. Even if each thread is running on a separate CPU it still requires operating system CPU cycles to get the threads running and put the unnecessary threads back into a blocking state.

Tymeac notifies each thread individually only when it is needed. By having a management structure for each Queue Thread, Tymeac knows the exact status of each Queue Thread so there are no wasted cycles notifying unnecessary threads.

Thread Pool
Tymeac's approach to the thread pool is to have only the minimum number of threads running at any given time so that Tymeac plays nice with other applications.

Tuning
Tymeac is tunable. Trying to manage traditional pool threads is like trying to herd cats.

Monitoring
Since Java threads execute at the application level there is no high level manager to supervise the environment. Tymeac uses a Monitor (daemon) at the application level that periodically scans the environment looking for problems.

Exceptions
When thread exceptions occur, Tymeac doesn't just let the system print an ugly message and kill the thread. Tymeac catches exceptions (including with an uncaught exception handler), prints and logs meaningful messages with full documentation, and notifies administrators of the problem.

Since most errors are with the user-written Classes (Developing Your Applications discusses this in detail), Tymeac allows dynamic repair and reload of those Classes with/without a GUI and restarting of the disabled threads with/without a GUI.

Tymeac controls threads the best they can be.

 

 

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