Motherboards with several video cards effects on hash rate

[pinjas]

Hello,

I've seen many people using risers and having 3-6 cards on a single motherboard.  However, every thread I've found, every listing I have seen where the collective hash rate for 3-6 cards, the average hash rate seems to be lower than expected.  Say you had 4 5850s, granted the number of mhash/s a person typically achieves varies from person to person, what I have found is around 380 per card is around the upper limit.  You would then expect a mhash/s rate of 1520.  However, I always see this to be not true.  I have seen this to be true with several multi-card setups.  Can anyone comment or state the reason behind this observation or maybe I am entirely mistaken.

I would have put this in the hardware section but I lack the necessary number of posts (totally understandable).

Thanks for keeping up this great forum folks, I hope to someday donate a few bitcoins towards it's upkeep.

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[Bert]

With 3-6 cards 0ne problem is cooling and another would be power. Think of 4 graphics cards running flat out as generating as much heat as a kettle constantly boiling (but with no water in  it). That heat needs to be removed, cooling is a problem. If you go over 4 cards then power is an issue, a 1.2KW PSU's may not have enough power for 5-6 graphic cards.

Around 380Mhash/second is the maximum that people are getting from a single 5850 card. Provided cooling can cope with it keeping each GPU under 85 °C (no idea what that is in F). And you have enough power, at 207W per GPU that is 828W. Allowing a very slim 50W for the motherboard, RAM, CPU and HDD/USB and 90% efficient PSU. That would mean that you need to need a 975 Watt PSU. 1KW PSU in reality with a 1.2KW PSU providing enough for a 5th card. Some PSU's have different outputs allowed for each half of the PSU, so it could be a power issue that is not allowing the GPU's realise their full potential.

If the cooling and power isn't there then the numbers will be lower, if the hardware can function. The GPU's will either start to cook at 95+ °C or if not getting enough power fail to work.

[blaznazn]

Normally, on setups like that, a not all of the video cards run at x16 mode.  Some will run x8 or x4.

[pinjas]

Normally, on setups like that, a not all of the video cards run at x16 mode.  Some will run x8 or x4.

Thanks for the responses bert and blaznazn.  My first thought was when seeing 3-6 card setups was, those motherboards must cost a small fortune to have that many x16 slots.  I looked into it, and it seems several individuals claim that for this, x1 vs x16 makes no difference.  The reasoning I can see this as a reasonable assumption/statement is that many also seem to 'know' that the ram on a video card is not important, it doesn't seem to matter how fast the ram is either as -everyone- underclocks- the ram, in many circumstances, they do so to radically low levels to generate less heat.  How I understand it, the number after the x in the x1 or x16 or whatever is a reference to the data transfer rate.  Bitmining doesn't load massive amounts of data, it requires massive amounts of calculations, I could be wrong, but that is what I've come to understand.

It sounds like it is being suggested that if you have enough power (easy to do) and are capable of keeping all of the cards cool enough, then all 4 cards should be running at their maximum. But these circumstances are seemingly never met?

[frutza]

If you have enough power, proper cooling and a decent motherboard the number of cards should not affect their performance. If power is insufficient, you can use multiple PSUs.

[CYPER]

With 3-6 cards 0ne problem is cooling and another would be power. Think of 4 graphics cards running flat out as generating as much heat as a kettle constantly boiling (but with no water in  it). That heat needs to be removed, cooling is a problem. If you go over 4 cards then power is an issue, a 1.2KW PSU's may not have enough power for 5-6 graphic cards.

Around 380Mhash/second is the maximum that people are getting from a single 5850 card. Provided cooling can cope with it keeping each GPU under 85 °C (no idea what that is in F). And you have enough power, at 207W per GPU that is 828W. Allowing a very slim 50W for the motherboard, RAM, CPU and HDD/USB and 90% efficient PSU. That would mean that you need to need a 975 Watt PSU. 1KW PSU in reality with a 1.2KW PSU providing enough for a 5th card. Some PSU's have different outputs allowed for each half of the PSU, so it could be a power issue that is not allowing the GPU's realise their full potential.

If the cooling and power isn't there then the numbers will be lower, if the hardware can function. The GPU's will either start to cook at 95+ °C or if not getting enough power fail to work.

1 - The performance table is now outdated, because it doesn't take into account the latest improvements in the miners. A 5850 can do 400Mhash/s.
2 - 5850 is in all cases under 200W, around 170W, so 4 cards = 750W in total.

[Bert]

If you have enough power, proper cooling and a decent motherboard the number of cards should not affect their performance. If power is insufficient, you can use multiple PSUs.

Care must be taken when using two PSU's to make sure that they are never directly interlinked. One supplying a motherboard, while another supplies hard disks is fine. But never connect two directly to each other, and they must use a common ground. Don't use one PCIe connection from each to the one video card.

[Wuked]

I'm using 3 x 5850's and here are the stats for them. They are in this order in my case ( so 2 runs the hottest )

There doesn't seem to be much performance difference between them

[frutza]

Cool graphs Wuked. Thanks!