Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

The performance difference between laptops and desktops is not really large today... with the exception of graphics cards. I was thinking of replacing my gaming desktop with a laptop, and using some external graphics card adapter to enable it to use a more powerful graphics card - for instance, one of Village Instruments' ViDock series, which connects using an ExpressCard interface.

However, I'm wondering - is there a performance impact of connecting a graphics card like that, via an ExpressCard, compared with the performance of connecting them via PCIe in a desktop computer? Is the bandwidth comparable? Is the latency comparable? etc.

share|improve this question
add comment

1 Answer

up vote 1 down vote accepted

Bandwidth:

"Good enough" is entirely subjective, and depends on what you wish to use it for. However, for a more objective view, we can compare the bandwidth. Wikipedia is as good a resource as any for this purpose.

PCIe page:

PCIe 2.x x16: 8 GB/s

[External ports are also possible with an] ExpressCard interface, which provides single lane v1.1 performance.

The page appears to be a bit outdated, since ExpressCard 2.0 provides about double that - close to single lane PCIe 2.x. See below.

ExpressCard page:

ExpressCard 1.2: 2.5 Gbit/s (PCI Express mode)

ExpressCard 2.0: 5 Gbit/s

PCIe 2.x x16 is 8 GByte/s, which is about 64 Gbit/s, which is over 10 times the bandwidth of ExpressCard 2.0.

The majority of high end video cards recommend PCIe 2.x x16. Therefore, the bandwidth will be lower then they require to run at full capacity. Does this affect performance? Yes. Does it appreciably affect performance? Depends what you want to do. I don't think newer 3D games will run well.


Latency

I couldn't find any direct information on this. However, I will say that latency is almost guaranteed to be higher - you have an additional cable, compared to a slot sitting on the motherboard. Whether this will affect performance at all (let alone in any appreciable way), I have no idea.

share|improve this answer
    
You prompted me to search Wikipedia some more, and I found this list, which puts PCIe x16 1.0 at 32 Gb/s and 2.0 at 64 Gb/s, and ExpressCard 1.2 at 2.5 Gb/s and ExpressCard 2.0 at 5 Gb/s. So it seems to agree with you that there's an order of magnitude difference here. –  Oak May 13 '12 at 7:57
    
@Oak Whoops, I missed the ExpressCard 2.0 info (didn't read the whole EC page). Thanks for the implicit reminder. –  Bob May 13 '12 at 8:10
    
Yep, but unfortunately for me, you were still very right in the spirit of things :) –  Oak May 13 '12 at 8:26
1  
There's a long list of benchmark results for different laptops and graphics cards here: forum.notebookreview.com/e-gpu-external-graphics-discussion/…. Comparing those to results from desktop systems should give a good idea on what kind of performance penalty to expect. Same forum has a ton of information for DIY implementations as well. –  Indrek May 13 '12 at 12:10
    
@Indrek incredibly useful link, thank you! –  Oak May 14 '12 at 7:38
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.