Strange... Both GPUs working at the same time? (2011 15")

Discussion in 'MacBook Pro' started by jimmie32, Oct 2, 2011.

  1. jimmie32 macrumors member


    Feb 11, 2011
    Beijing, CN
    First, some starting info:
    I mine bitcoins, using a OpenCL/GPU Miner (Diablo), on my 15" 2.3/6750M system. It works well and yields pretty average hashrates, but if I turn the GPU mining on, it will start lagging my screen draws like hell, basically I can't run GPU mining while using the MBP.

    I thought for a second I could use the Intel HD Graphics for normal operations and exclusively dedicate the 6750M GPU to the miner. Of course, it didn't seem possible for me, so I didn't further research it.

    One day (I mean today), I decided to switch my system to Integrated Only (using gfxCardStatus), just to see how the miner would react. I am aware that there's partial OpenCL support in the CPU (not in the HD 3000 though), and so I decided to launch the miner with Integrated Only...

    Note that GPU mining usually gives me 55000 or so khash/sec, and CPU peaks out at 4000 khash/sec or so on lucky days. So the difference would be obvious if I ran the GPU/OpenCL miner on a CPU's OpenCL engine.

    I expected to see a 4000 khash/sec or so in my GPU miner.
    Instead, what I was was 61000 khash/sec speeds and *no lag* in the UI at all. gfxCardStatus keeps showing "Integrated Only".
    GPU miner works perfectly at even higher speeds, the UI isn't lagging, and all apps work correctly, including video (woo!), which is very surprising to me.

    I don't have external monitors hooked up as I don't need one, but this is strange to me. I mean, I thought that the MBP couldn't possibly use *two* GPUs at the same time, even if one was handing screen and the other OpenCL. I mean, I used gfxCardStatus to set to Integrated Only. It shouldn't even be supposed to trigger the 6750M anyway. But it did, and allows me to sort-of use both. Even though its for different purposes.

    Can anyone confirm this is already known or is this something new? I think the former would be more possible, but since search didn't show anything I decided to write this up.

    tl;dr: I can run a OpenCL/GPU Mining App on the 6750M *while* driving the internal display with *only* the Intel HD 3000.

  2. dusk007 macrumors 68040


    Dec 5, 2009
    I must admit I don't know too much about mining but Intel put quite a bit of very efficient fixed function stuff into the GPU. Might be it works quite well for mining.
    One should test it on a 13" MBP where there is no 6750M around to see what the score of the Intel GPU is.

    Could be also a bug and it wasn't intended to work that way.

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