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The Race Is On to Debug Dual-Core Deadlocks


Exploring best practices for building parallel applications

By Jennifer deJong
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February 1, 2006 —  (Page 1 of 3)
You write a program. You test it. You ship it. Then, mysteriously, it doesn’t run.

What’s the culprit? Most likely, a deadlock or a race condition, said James Reinders, a senior engineer at Intel. Both errors can grind a multithreaded application to a halt, and are notoriously difficult to detect. “They simply don’t exist in sequential programs,” he said.

Deadlocks and race conditions aren’t new; they’ve afflicted multiprocessor systems, particularly large servers, for many years. But dual-core processors for the desktop, delivered by AMD and Intel last year, are bringing them to the fore. That’s generating renewed interest in an old problem: the difficulty of designing and coding applications that execute multiple processes concurrently.

Dual-core processors don’t fundamentally change the way developers write applications, said Margaret Lewis, director of commercial ISV marketing at Sunnyvale, Calif.-based chipmaker AMD. But they further enable multithreading—also known as concurrent development or parallel programming—in which an application sends “multiple execution strings to the processor,” she said.

Threads within multithreaded applications, which also can run on single processors, often compete for shared resources (such as memory and disk space) as well as variables and functions. A deadlock can occur, for example, when a multithreaded application locks a resource and fails to unlock it once the thread has completed its task, said Andy Yang, product manager at

San Francisco-based Coverity, which sells a source code analyzer, among other tools. “The system freezes up.”

Race conditions happen when threads compete for the same resources and the application fails to specify the order in which the threads can access those resources. “You solve the problem with synchronization,” said Reinders. “If the flag is up, don’t take [the resource]. If the flag is down, it’s available.”

Detecting either error is difficult because collisions don’t always occur, so pinpointing the problem is a matter of timing, said Ken Cowan, a product line manager for Detroit-based Compuware, which sells coding, testing and other tools. When QA tests the app by hand, the error doesn’t appear, but it may show up in load testing, he said. “With 50 users, the application fails. And it’s not clear why.”




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