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Symmetric Multiprocessing: Performance without the Power (Consumption) Print E-mail
Written by SymbianOne   
Sunday, 30 November 2008
Future generations of Symbian OS phones will be harnessing the performance offered by multiprocessor technology. Jason Parker, Senior Product Manager at Symbian explains the philosophy behind Symbian's approach and the impact of this technology on application developers.

The web is becoming pervasive. Mobile networks are increasing their capacity to offer high speed and affordable data. As a result, users of mobile devices have growing expectations that they can access the web wherever they are and whenever they want.

"One of the challenges with the mobile web is that users want to browse the same pages they use on their desktop PC on a mobile device. However, a lot of web content relies on the processing power of a desktop to ensure a good user experience," says Jason. "Take a typical mySpace home page as an example. It will probably include a number of plug-ins for media that have been designed for a desktop environment. As a result, a PC may easily use half its CPU capacity to display some seemingly trivial item of content. With Symmetric Multiprocessing (SMP) it becomes possible to render this non-optimized or barely optimized content on a smartphone and offering a good user experience."

These issues arise with content such as Flash Video, which uses the proprietary On2 software codec and not H.264 video. H.264 video can already be processed power efficiently in dedicated video processors and Jason does not see SMP technology changing this. For non-optimized content, however, SMP is the answer. It offers the performance to render the content fast enough to give the user a browsing experience similar to the desktop.

SMP is a relatively old technology, which has been gravitating from mainframes to ever smaller computing platforms. Generally its implementation has been about achieving performance and throughput, with little regard to power consumption.

At the same time, in smartphone design, technology gains in battery performance are often offset by a demand to reduce component size. This means that the power available to a smartphone has tended to remain constant.

"Our approach to implementing SMP is different from other operating systems," says Jason. "We have sought to optimize the benefits of SMP from both power management and computational performance perspectives."

The ability to meet these apparently conflicting demands has been achieved by optimizing Symbian OS to take advantage of the hardware features of ARM's SMP processor technology.

"The hardware designs allow us to turn CPUs on and off in microseconds, rather than the milliseconds or seconds needed to perform the same task in a desktop or server," says Jason. "These are similar times to context switching in a single CPU system, which allows Symbian OS to respond as quickly to changes in processing requirement in an SMP environment as it can in a single CPU environment. By aggressively managing the CPUs Symbian OS can deliver bursts of very high performance when needed, while reducing the power used by the device when it is idle by turning off processors. The result is that we place similar demands on overall battery life compared to today's technology."

"Symbian OS has always been well placed to take advantage of multiprocessor technology," says Jason. "It relies on a large number of servers and is highly threaded. If you examine a Symbian OS device after it has been booted you might see 100 or more servers and many more threads running. This provides an ideal foundation for spreading load over multiple processors."

Although not an objective for SMP in Symbian OS, the performance headroom offered by SMP may allow device manufacturers to reassess their hardware designs. For example, they may be able to move processing from dedicated DSP components to a general computing task. "With SMP device manufacturers will get a lot more freedom in their hardware designs," says Jason. "It probably won't have an immediate impact: designers will want to explore the best ways to optimize their designs with the new technology. But over the next 18 months to 3 years I expect it will start to have an impact on their bill of materials."

So what does this mean for third-party developers? "If developers are already using good design practices, we don't expect they will need to make any special considerations to take advantage of SMP," says Jason. Good design patterns that will work well with SMP include making use of asynchronous APIs, buffering schemes, and using multiple threads to subdivide long tasks.

"Many native Symbian applications pass a lot of their workload to system servers," says Jason. "SMP will offer an immediate benefit to these applications because it can distribute the computational load from these servers, making the applications more responsive and thereby providing an even better user experience."

Where developers want to move computationally intense applications to Symbian OS they may need to make more specific design decisions to get the most out of SMP. "Developers will have to work upfront to design algorithms and data structures so that the workload can be subdivided into multiple threads," says Jason. "This is, however, a fairly standard software engineering practice and there are no particularly novel requirements created by SMP."

Jason points out, however, that developers will need to pay particular attention to shared data structures. "Make sure they're well protected," says Jason. "Without suitable locks, when you've got multiple processes working with shared memory, it is very easy to get unexpected results."

A typical example of the type of application where SMP will have a big impact is route planning. "Today many applications do route planning off-device, or do it quite slowly on the device, because it's very computationally intense," says Jason. "With SMP technology in a Symbian OS device, offering that burst of performance, a task like this could be done eight or 10 times faster."

"It's been a very exciting project," says Jason. "There have been huge technical challenges in turning CPUs on and off and winding their performance levels up and down. But equally it's been very satisfying seeing how Symbian OS can provide a very broad spectrum of operating capacities: From handling intense computations and then going back down to an idle state consuming very little power."

Jason expects dual processor Symbian OS phones to be appearing in the market in 2010/11.

For more information see http://www.symbian.com/symbianos/os_smp.html.



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