Hash Rate Drops with 2nd 280x

[richmke]

I have been mining for 2 months with a Gigabyte 280x GV-R928XOC-3GD REV2. I undervolted and got it to mine at 710-730 Khs. I am using an Athlon II X3 processor with a Gigabyte motherboard that has two x16 slots. I have a Antec CP-850 PSU. More than enough power for two cards.

I bought a 2nd GV-R928XOC-3GD REV2, configured it the same, and it also mines at 710-730 Khs when only the one card is mining.

If I have both cards mine (two separate instances of cgminer), the hash rate drops to 680-700 Khs for each card (both cards running at 80c). I have two instances because I want the pool to track them as separate miners so I can fine tune each card.

Any thoughts on why the per card mining rate drops when both cards are mining?

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[spiderdk]

cool them.., vrm temps are high?

I get that on a rig almost same as you. Added a cooler near it and problem solved.

Also, unplug your monitor, remote connection to get more stable results.

[phrozenspite]

I have been mining for 2 months with a Gigabyte 280x GV-R928XOC-3GD REV2. I undervolted and got it to mine at 710-730 Khs. I am using an Athlon II X3 processor with a Gigabyte motherboard that has two x16 slots. I have a Antec CP-850 PSU. More than enough power for two cards.

I bought a 2nd GV-R928XOC-3GD REV2, configured it the same, and it also mines at 710-730 Khs when only the one card is mining.

If I have both cards mine (two separate instances of cgminer), the hash rate drops to 680-700 Khs for each card (both cards running at 80c). I have two instances because I want the pool to track them as separate miners so I can fine tune each card.

Any thoughts on why the per card mining rate drops when both cards are mining?

the r9 series are notorious for downclocking themselves in terms of temp, that's about all i can think of. 

[richmke]

I don't know how to get vrm temps. gpu1 is sitting at 74c but only hashing at 680 Khs.

I did reduce the intensity on gpu0 to drop its temp to 77c with a hash rate of 425 Khs. I am getting an x16 extension so I can lift gpu1 out of the intake airflow of gpu0. At that point, they should both be able to run at full intensity around 75-77c. Right now, if I run gpu0 at full intensity, it will heat up to 90+c while hashing at 700 Khs.

[spiderdk]

blow cool air to see if thats your solution, than you thin about something else to cool it properly

[Equate]

This is called as throttling as the new bios of the cards are not good, either you have to flash the bios or to keep the cards below 75 degree.