a question on Gpus collective performance

[ihmoal]

the question goes, when you increase the number of GPUs on your one motherboard, does the HASHING SPEED multiply or does it add up ??

Example: suppose the hashing speed for GPU of Radion 5830 is 300 Mhash/sec, when you add another 2 GPUs, what would be the total speed ? is it A. 300+300+300=900 or B. 300*2*2=1200

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here is the source of my question :
quote from the following link:
http://erratasec.blogspot.com/2011/06/password-cracking-mining-and-gpus.html

"But, the thing to keep in mind is decreasing marginal returns. Buying a $250 Radeon card will increase cracking speeds by 20 times. Buying a second $250 Radeon card will only double the previous card’s speed. A single desktop with four Radeon HD 6990s for $3000 will increase cracking speed by 160 times. Buy a second such system, for another $3000, will only double your cracking speed after that."

[Gabi]

Uh? Of course each GPU simply add his performance.

If you have a gpu that make 300mhash/s you have a total of 300mhash/s

If you have 3 gpu, you will have a total of 300+300+300, so 900mhash/s


You are misunderstanding the article. If you have 1 gpu and you buy 2 more, your total mhash of course triplicate, cause well from 300 it become 900. If you buy 3 more gpu of course it will only double cause it will become 1800mhash/s. But 1800 is 6 times 300! So 6 gpus are 300+300+300+300+300+300