Re: Three GPU cards on shared-mem motherboard

From: Axel Kohlmeyer (
Date: Mon May 28 2012 - 11:20:13 CDT

On Mon, May 28, 2012 at 12:03 PM, Francesco Pietra
<> wrote:

>> When referring to NAMD, I wanted to imply (badly, I admit) performance
>> boost by the third GPU.

as i was mentioning before. that is near impossible to predict.

>> The 990FXA-GD80 PCI specification is described by the manufacturer as follows
>> -- PCI Express slots version: 2.0.
>> -- PCI slots: 1.
>> -- PCI express x1 slots: 1.
>> -- PCI express x16 slots: 4.

that doesn't mean anything. labeling slots as x16
only means that you can stick an x16 wide card
into it. each of these slots can be wired with 16,
8, 4, 2 or 1 lane. also, some boards claim they
have all 16-lane slots, but then two slots are
connected to a little bridge chip. resulting in
two cards each having to share the bandwidth.

>> Whether these are real x16 2.x, or not, is beyond my understanding. I

with out that information, you can't judge.
contact the vendor or find somebody that
has time to research it.

>> can only compare with the corresponding description for the mainboard
>> I am currently using: GA-890FXA-UD5:
>> 2 x PCI Express x16, running at x16 (PCIEX16_1, PCIEX16_2).
>> 1 x PCI Express x16 slot, running at x8 (PCIEX8).
>> 1 x PCI Express x16 slot, running at x4 (PCIEX4).
>> 2 x PCI Express x1 slots.
>>  (All PCI Express slots conform to the PCI Express 2.0)
>> 1 x PCI slot.
>> With this latter mainboard, adding a second GTX-580 gave the expected
>> acceleration. Data for PCIs of the two mainboards being comparable, I
>> would expect that a third GTX-580 on the 990.. motherboard should play
>> well its job. Is it this naive extrapolaion a sound one?

no. you usually overload the memory bandwidth of the CPU
with the third GPU and thus you won't get the full speedup.
how much speedup you'll get depends on the individual
characteristics of your input.


>> Thanks indeed for further advice
>> francesco pietra

Dr. Axel Kohlmeyer
College of Science and Technology
Temple University, Philadelphia PA, USA.

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