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External Graphics Performance: GPU Compute

Echo Express Pro: Desktop Graphics In A Thunderbolt Chassis
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Sonnet Technologies' chassis wasn't designed for graphics cards. It doesn't expose six- or eight-pin auxiliary power connectors. So, you either need to use entry-level graphics hardware like the Radeon HD 7750, or be willing to run a couple of leads from your PC's power supply. Naturally, that doesn't map over to notebooks, but it still makes for an interesting experiment.

Although we know what the Echo Express Pro was meant to do (enabling performance-oriented expansion for storage, A/V, networking, and so one), dropping a graphics card into the enclosure is way cooler. Most of our excitement stems from the fact that notebook graphics potential is highly dependent on the thermal, mechanical, and electrical constraints of a mobile platform. Taking that outside of the box, so to speak, opens the door to more flexible configurations.

Nowadays gaming isn't the only application for GPUs, though. The breadth of GPU-accelerated software is widening, including compression algorithms, video editing, image editing, and even bitcoin mining. Many of these titles are able to fully utilize a graphics processor's resources, consequently putting more stress on the interconnects linking each subsystem. How big of an impact does externalizing a four-lane PCI Express link have on compute performance?

CLBenchmark is a new tool created by our friends at Kishonti, also known for their GLBenchmark program for iOS and Android. It is similar to LuxMark in that it can test ray tracing and image manipulation; however, we are specifically focusing on its SPH Fluid Simulation benchmark.

As you'd expect, the card plugged straight into a motherboard performs better. But the magnitude of the impact depends on the GPU's performance, it seems. AMD's Radeon HD 6970 runs 14% faster plugged straight in to a 16-lane PCI Express slot, whereas Nvidia's GeForce GTX 460 is only 5% faster. If you're looking to upgrade a notebook with mainstream card without auxiliary power connectors, it's probable that Thunderbolt won't negatively affect its potential much at all.

LuxMark tells us a similar story. The Radeon HD 6970 takes a much larger hit than Nvidia's GeForce GTX 460. The 6970 is not particularly realistic in this application, though, given its power requirements. Boards with more conservative power needs should perform almost as if they were running in a native x16 slot, even if you attach them via Thunderbolt using Sonnet Technologies' chassis.

Diagnostics like SiSoftware's Sandra specialize in isolating specific subsystems. This title's general-purpose processing module effectively factors out interface bandwidth as a variable, putting most of its emphasis on the GPU. As a result, it doesn't really matter if you operate over a native 16-lane link or Thunderbolt's 10 Gb/s channel.

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