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Author Topic: different hashing rates with 1 card vs. 2 (5830)  (Read 1066 times)
CanaryInTheMine
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July 26, 2011, 05:22:01 PM
 #1

Can someone explain why 1 card (sapphire 5830 extreme) is capable of 333 MHashes at 1040/350 60% fan 62 degrees but if I plop a second card in, at best I can get 1000/350 on one and about 930/350 on the second.
750W cooler master RS-750-ACAA
I don't believe it's a power issue (tried 2 different supplies although same Wattage) my second thought is mobo is contributing to this difference... ?

has anyone experimented with something like this or has seen same behavior by any chance?

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Roland68
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July 26, 2011, 05:25:54 PM
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Same on my Asus P5Q mobo ...

I will change power supply this week , hoping the second card get his speed ...

switched the cards doesn't change ...the second card get slower speed ...

strange!
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July 26, 2011, 05:41:32 PM
 #3

Can someone explain why 1 card (sapphire 5830 extreme) is capable of 333 MHashes at 1040/350 60% fan 62 degrees but if I plop a second card in, at best I can get 1000/350 on one and about 930/350 on the second.
750W cooler master RS-750-ACAA
I don't believe it's a power issue (tried 2 different supplies although same Wattage) my second thought is mobo is contributing to this difference... ?

has anyone experimented with something like this or has seen same behavior by any chance?

There's additional heat produced, the additional electrical load on the motherboard as well as EMI can affect stability. It's like boards with high capacity memory DIMMs, it's not unusual to find the manual warning that if you use more then X dimms with Y capacity, you can only run the memory at a lower speed.

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CanaryInTheMine
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July 26, 2011, 05:56:22 PM
 #4

Can someone explain why 1 card (sapphire 5830 extreme) is capable of 333 MHashes at 1040/350 60% fan 62 degrees but if I plop a second card in, at best I can get 1000/350 on one and about 930/350 on the second.
750W cooler master RS-750-ACAA
I don't believe it's a power issue (tried 2 different supplies although same Wattage) my second thought is mobo is contributing to this difference... ?

has anyone experimented with something like this or has seen same behavior by any chance?

There's additional heat produced, the additional electrical load on the motherboard as well as EMI can affect stability. It's like boards with high capacity memory DIMMs, it's not unusual to find the manual warning that if you use more then X dimms with Y capacity, you can only run the memory at a lower speed.


I was under the impression that memory speed is not important for mining.

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July 26, 2011, 06:04:34 PM
 #5

I was under the impression that memory speed is not important for mining.

It's not. The memory DIMMs on the motherboard example was just to illustrate how having additional loads (electrically speaking) can reduce the electrical stability of the subsystem, forcing operations to a slower speed or risk errors. So having additional cards inserted can cause similar problems, in addition to the obvious heat being produced, resulting in lower max O/C ceiling.

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CanaryInTheMine
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July 26, 2011, 06:08:26 PM
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I was under the impression that memory speed is not important for mining.

It's not. The memory DIMMs on the motherboard example was just to illustrate how having additional loads (electrically speaking) can reduce the electrical stability of the subsystem, forcing operations to a slower speed or risk errors. So having additional cards inserted can cause similar problems, in addition to the obvious heat being produced, resulting in lower max O/C ceiling.

Ambient heat or heating of the mobo components?

So, the mobo is likely culprit.  What motherboards are rock solid that have at least 2 PCIe slots for GPUs and have ample spacing between them?
Currently i'm using the MSI 870A-G54 mobo

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July 26, 2011, 06:15:10 PM
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Ambient heat or heating of the mobo components?

Both are contributing factors. Component temperatures are always ambient + generated dissipation.
Additional electrical loads can pose very marginal heat issues but cause significant electrical loading problem.


Quote
So, the mobo is likely culprit.  What motherboards are rock solid that have at least 2 PCIe slots for GPUs and have ample spacing between them?
Currently i'm using the MSI 870A-G54 mobo

Additional load causing electrical interference is pretty much unavoidable. I doubt any motherboard can completely eliminate the problem. On server boards with a lot of DIMMs, we have to use buffered DIMMs running at slower latencies. So changing a board might not get rid of the problem, and even if it does, is the MHash difference enough to cover the cost in a reasonable amount of time?

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July 26, 2011, 06:20:41 PM
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Some premium motherboards like MSI BB Marshal can absorb massive electric loads without affecting the stability of addons on the mobo.
The downside is that those boards cost a lot ($400 or more) due to having high quality components, capacitors, chokes etc.

Yes, a low quality board can limit the overclocking capability of a card if a lot of power is being drawn through too many slots (75W for each card), at some point the power draw will choke and some cards might be getting sub-par power.

One way to circumvent this is using modded extender cables w/ Molex which feed the card the 75W from the PSU rather than the motherboard.

Quote from: CanaryInTheMine
but if I plop a second card in, at best I can get 1000/350 on one and about 930/350 on the second.

Try OC'ing them individually. If the problem persists then those are the hard limits of the cards you have, i.e they can't be pushed higher.

I have some 5850's that wont go even a bit above 880mhz, and some will go easily past 1ghz.

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CanaryInTheMine
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July 26, 2011, 08:19:59 PM
 #9

Some premium motherboards like MSI BB Marshal can absorb massive electric loads without affecting the stability of addons on the mobo.
The downside is that those boards cost a lot ($400 or more) due to having high quality components, capacitors, chokes etc.

Yes, a low quality board can limit the overclocking capability of a card if a lot of power is being drawn through too many slots (75W for each card), at some point the power draw will choke and some cards might be getting sub-par power.

One way to circumvent this is using modded extender cables w/ Molex which feed the card the 75W from the PSU rather than the motherboard.

Quote from: CanaryInTheMine
but if I plop a second card in, at best I can get 1000/350 on one and about 930/350 on the second.

Try OC'ing them individually. If the problem persists then those are the hard limits of the cards you have, i.e they can't be pushed higher.

I have some 5850's that wont go even a bit above 880mhz, and some will go easily past 1ghz.

individually they OC very well...  i really think it's the mobo I have... now, if both cards are powered up, the system reboots sporadically (and yes, I've eliminated the PSU as the possible cause or reboots).  Oh well... new mobo ordered...
I do agree that spending a ridiculous amount to achieve same OC on all cards in the system isn't worth the extra money.  you can practically build another system at that point to get more MHashes.

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CanaryInTheMine
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July 28, 2011, 10:12:30 PM
 #10

swapped the mobo.  no more sporradic reboots (so far anyways)

had a 870A-G54 replaced with 870-G45

I can now OC the 2 sapphire 5830 extreme cards to 1040/350/60% fan and 970/350/75% temps are upper 60ies.

I've noticed that if the speed of the card is the 16x PCIe it OCs lower?  
This is just an observation right now.  Will swap out 2 XFX 5830s to 2 5830 sapphire extremes on my last rig and take a peek at the lane speeds and OC capability in a few hours.  will report what I find out.
if this holds true, will try to figure out if there's a way to "downstep" the PCIe lane speeds...

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Roland68
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August 04, 2011, 01:06:08 PM
 #11

Changed my psu with a 850w corsair, AND feeding the mobo with the two power input (asus p5q pro) doesn't allow the second card to get his full power, and full speed  ...swapping the card ... the same ... must be mobo design ...

sad ...
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August 04, 2011, 04:14:22 PM
 #12

Mobo's definitly contribute to this issue.

I wonder if it's the mobo's ability to draw power to that slot or if it's bus timing.

to test if it's power draw related, the second card, the one that isn't able to reach higher speeds, a cable extender with molex (power) should be used.  If it reaches expected speeds, then it's power issue through mobo to that specific pcie slot.

otherwise, it's something to do with bus timing.  In this case, maybe under-clocking CPU and memory (if possible) can be explored to see if anything can be improved...

I'm not sure what else to consider here...

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