Friday, October 07, 2005

The joys of symmetric multiprocessing

In my End of the Free Lunch post, I mentioned how desktop CPUs are moving toward a multiprocessor architecture that will require major changes in the way programmers write software in order to attain higher performance. Well, this isn't just a trend in the desktop market. The embedded systems market is also seeing a proliferation of multi-CPU devices. At first, they were exotic boards that used parallel arrays for signal processing. Then, you saw hybrid systems with two chips (sometimes in the same package), commonly used in cell phone applications. Now you are seeing rather standard architectures with dual cores, like a recently announced PowerPC chip for military, telecom, and other embedded markets.

What does this mean for embedded systems? Well, opinions seem to vary. Enea encourages asymmetric multiprocessing (i.e. an OS running on each chip and different programs running on different chips) in their article "Who's afraid of asymmetric multiprocessing"

In addition, SMP is not well suited for many applications that require predictable, real-time response. It does not scale well to very large systems, and it lacks fault-tolerant features.

Firing back in the same magazine, QNX says:

By running only one copy of the OS, SMP can dynamically allocate resources to specific applications rather than to CPU cores, thereby enabling greater utilization of available hardware. It also lets system tracing tools gather operating statistics and application interactions for the multi-core chip as a whole, giving developers valuable insight into how to optimize and debug applications.

So whom do you believe? Well, it probably depends upon your application and your viewpoint on the world. Coming from a developer on a huge code base, I'm firmly in the SMP camp. Why? Well, first, LabVIEW already runs well in an SMP system. We put in a lot of work to make our application nicely multithreaded and I want our customers to be able to take advantage of it. The second is that moving an application from a multithreaded environment to an asymmetric multiprocessing system (AMP) requires a change in architecture. Changes in architecture are hard... Really hard. So I'm not "afraid" of AMP, but I'm certainly "annoyed" that I'd have to contemplate it.

Yes, SMP will never work across two different types of processors. That's what we've got LabVIEW 8 with Distributed Intelligence for...

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