Common Errors in C

A nice link I had wished to see earlier about programming in C. The author has also a tutorial on assembler.

Java Workqueue API

As I explained in a previouse entry, since java 1.5 there are new classes and interfaces to help the implementation of parallel systems. These structures are found in the java.util.concurrent package. But how are these structures to be used. The package description recalls the main types of interfaces and classes in this package:

• executors
• queues
• concurrent collections
• synchronizers
• atomic (in package: java.util.concurrent.atomic )
• locks (in package: java.util.concurrent.locks )

Of course, the structures discussed in this package have somewhat similar to the one found in typical kernel discussions: work queues and semaphores... Let us now take a look at the different parts.

Executors

Executors are containers taking care of the execution of tasks, just as SwingUtilities used to do it. Different kinds of executors are imaginable:

• DirectExecutor (performing the task but not asynchronously)
• ThreadPerTaskExecutor (one thread is created for each task to be performed)
• SerialExecutor (each task is performed sequentially)
• ThreadPoolExecutor (executor performing using a pool of threads)
• ScheduledThreadPoolExecutor (executor performing using a pool of threads at certain specified moments or periodically)

The implentations of the java.util.concurrent package implement the ExecutorService interface which provides a number of supplementary methods.

Queues

Another useful data structure for performing task in parallel and asynchronously are queues (also known as FIFO data structures). The java.util.concurrent package provides a number of data structures for this purpose too. One particular kind of FIFO are blocking queues, for which five different versions exist:

• LinkedBlockingQueue
• ArrayBlockingQueue
• SynchronousQueue
• PriorityBlockingQueue
• DelayQueue

The first two are queue data structure backed by a linked list, respectively an array. The synchronous queue is special in the way that the queue accept elements only if at the same times elements are removed from the queue on the other end (though I must admit I still need to look at how this actually performed). The priority blocking queue is a FIFO with priority to use the same ordering as a PriorityQueue. Finally, the delay queue is a blocking queue only providing elements when a certain delay as passed for the element to be retrieved.

Synchronizers

A number of possible techniques can be used to synchronize threads. Among these are semaphores which we discussed in the linux kernel context. Other types provided by the java.util.concurrent package, such as the CountDownLatch, used to block some actions until a number of events or elements have been counted, the CyclicBarrier, which is a resettable multiway synchronization mechanism, an exchanger to allow threads to exchange objects at some definite point and the already mentioned locks.

Concurrent Collections

The java.util Collection Framework already contained a number of snychronized or synchronizable classes. However, the java.util.concurrent package introduces new structures useful in a multithreaded context:

• ConcurrentHashMap,
• ConcurrentSkipListMap,
• ConcurrentSkipListSet,
• CopyOnWriteArrayList, and
• CopyOnWriteArraySet

The synchronization approach of the usual synchonized classes came with a drawback for the scalability, because they imposed a strong check on the serializability of the action performed on the data structure. The Concurrent implementations are on contrary not as restrcited. It remains in the job of the developper to know when to prefer which implementation.

Timing Units

The java.util.concurrent package also introduces a new class TimingUnit to provide different kind of granularity as to which unit of interval for time measurements should be used. Here again, it would be useful to take a look at the implementation of the kernel and compare.

Atomic and Locks

Since atomic variables and locks are in other packages, I will describe them in other entries. However, it is again interesting to note that the same topics were already discussed in other entries of this blog.

Flash with Flex

So I tried a few things with Flash now that I more or less know the basics of flex (and only the basics).

So I wrote my first little movie in flash and compiled it using flex's compiler.

I can not really do an introduction yet to flash and flex. I still need to get use to too many things. For example, I am not really happy not knowing which data structures are available in action script. Maybe I have been spoiled by java using the java.util collections. Yet I think it sensible to expect the existence of a number of standard libraries one can use to prevent the inventingthewheelitis.

One thing I found at least im internet is from a developer from Mannheim who wrote some data structures for games in action script. I will have to take a look at it. It sure sounds really interesting.

I have also been interested in what is the best way to create small movies for fun. So I just thought of the general object oriented structure of my character creations. Actually I already had some project of the sort in java. But I had not that much because other priorities popped up as they always do.

Oprofile - What are thou ?

O.K Apart from doing stupid references to films I actually have not really liked. What am I doing?

After reading a few mails from the Linux Kernel Mailing list, I found the following tool which seems useful: oprofile. I must admit I still do not have a clear idea of all the possibilities that this tool offers.

The first thing to know it is a profiler and it is a profiler capable of giving a number of information on a program with quite a low overhead. But what is a profiler?

A profiler collects information on running processes of a computer to analyze the performance of a program (see the wikipedia article on performance analysis).

It gives the possibility to look at the following aspects of a system:

• Call-graphs (i.e it looks at the graph of the functions calls of programs)
• libraries used by a program
• instruction level information collections (also on assembly code level)

I will probably continue to take a few look at the possibilities of this tool in the next few weeks.