[clug] Modern laptops, nVidia, Optimus and Linux

Alex Satrapa grail at goldweb.com.au
Wed May 4 20:20:03 MDT 2011

On 05/05/2011, at 11:03 , Hugh Fisher wrote:

> Putting the graphics card in the monitor would increase the
> bandwidth required, not decrease it. PCIe 2 with 16 lines
> is 64Gbits/sec, DisplayPort only needs 17Gbits/sec. Some
> graphical/number crunching programs prefer having two cards
> available.

Is that due to PCIe bus limitations or limitations of the processing power of one graphics card?  I can't see that adding a second card is going to solve problems of bus congestion, just like adding more cars to the road isn't going to solve road congestion.

A Thunderbolt equipped Mac can drive a 30" Cinema Display (only one). That's two HD streams worth of data over one Display Port-over-Thunderbolt link, and the limitation is due to DP-over-TB operating at about 10Gbps instead of the 17.2Gbps it's theoretically capable of.  The 27" iMac has two Thunderbolt ports, and can connect to two external displays (including using the screens of two other iMacs via Thunderbolt interconnect).

IIRC, AGP was only about 0.5Gbps bandwidth, PCIe x1 is about 4Gbps. Based on those dodgy figures, you should be able to run several AGPx2 or x4-style external graphics cards per Thunderbolt port. If you're not running high demand services (e.g.: you're just expanding someone's GIS desktop), I don't see that you'd really need that much bandwidth. I don't see that piping OpenGL commands and textures over the bus will use much bandwidth when all you're doing is GIS, web browsing and word processing.


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