Sunday, May 4, 2008

Multiprocessing: How 'bout that Free Lunch?

I remember reading an article, a few years back...

The Free Lunch Is Over: A Fundamental Turn Toward Concurrency in Software

Its tagline: "The biggest sea change in software development since the OO revolution is knocking at the door, and its name is Concurrency."

Mr. Sutter's article suggests that because CPUs are now forced to improve performance through multi-core architectures, applications will need to typically employ multi-threading to gain performance improvements on newer hardware. He made a great argument. I remember getting excited enough to bring up the idea to my team at the time.

There are a number of reasons why the tag line and most of its supporting arguments appeared to fail, and in retrospect, could have been predicted.

So in today's age of multi-core processing, where application performance gains necessarily come from improved hardware throughput, why does it still feel like we're getting a free lunch?

To some extent, Herb was right. I mean, really, a lot of applications, by themselves, are not getting as much out of their host hardware as they could.

Before and since this article, I've written multi-threaded application code for several purposes. Each time, the threading was in UI code. The most common reason for it: to monitor extra-process activities without blocking the UI message pump. Yes, that's right... In my experience, the most common reason for multi-threading is, essentially, to allow the UI message pump to keep pumping while waiting for… something else.

But many applications really have experienced significant performance improvements in multi-processor / multi-core systems, and no additional application code was written, changed, or even re-compiled to make that happen.

How?


  • Reduced inter-process contention for processor time
  • Client-server architectures (even when co-hosted, due to the above)
  • Multi-threaded software frameworks
  • Improved supporting hardware frameworks
Today's computers are typically doing more, all the time. The OS itself has a lot of overhead, especially Windows-based systems. New Vista systems rely heavily on multi-processing to get performance for the glitzy new GUI features.

The key is multi-processing, though, rather than multi-threading. Given that CPU time is a resource that must be shared, having more CPUs means less scheduling collision, less single-CPU context switching.

Many architectures are already inherent multi-processors. A client-server or n-tier system is generally already running on a minimum of two separate processes. In a typical web architecture, with an enterprise-grade DBMS, not only do you have built-in “free” multi-processing, but you also have at least some built-in, “free” multi-threading.

Something else that developers don’t seem to have noticed much is that some frameworks are inherently multi-threaded. For example the Microsoft Windows Presentation Foundation, a general GUI framework, does a lot of its rendering on separate threads. By simply building a GUI in WPF, your client application can start to take advantage of the additional CPUs, and the program author might not even be aware of it. Learning a framework like WPF isn’t exactly free, but typically, you’re not using that framework for the multi-threading features. Multi-threading, in that case, is a nice “cheap” benefit.

When it comes down to it, though, the biggest bottlenecks in hardware are not the processor, anyway. The front-side bus is the front-line to the CPU, and it typically can’t keep a single CPU’s working set fresh. Give it a team of CPUs to feed, and things get pretty hopeless pretty quick. (HyperTransport and QuickPath will change this, but only to the extent of pushing the bottle necks a little further away from the processors.)

So to re-cap, to date, the reason we haven’t seen a sea change in application software development is because we’re already leveraging multiple processors in many ways other than multi-threading. Further, multi-threading options have been largely abstracted away from application developers via mechanisms like application hosting, database management, and frameworks.

With things like HyperTransport (AMD’s baby) and QuickPath (Intel’s), will application developers really have to start worrying about intra-process concurrency?

I throw this one back to the Great Commandment… risk management. The best way to manage the risk of intra-process concurrency (threading) is to simply avoid it as much as possible. Continuing to use the above mentioned techniques, we let the 800-lb gorillas do the heavy lifting. We avoid struggling with race conditions and deadlocks.

When concurrent processing must be done, interestingly, the best way to branch off a thread is to treat it as if it were a separate process. Even the .NET Framework 2.0 has some nice threading mechanisms that make this easy. If there are low communications needs, consider actually forking a new process, rather than multi-threading.

In conclusion, the lunch may not always be free, but a good engineer should look for it, anyway. Concurrency is, and will always be an issue, but multi-core processors were not the event that sparked that evolution.
Post a Comment