Power limit doesn't affect GPU's performance?

3

My mining rig's GPU each draws approximately 150 watts with an average hash rate (it's performance level) of 29 M.hash/s. While I was tweaking it's settings, I limited its max power draw to 135 watts and expected the GPU to throttle down, but it did not. Its power draw does reach the limit of 135 watts, then stop, while maintaining its hash rate of 29 M.hash/s (clock speed didn't drop). Wonder how could that be happening, I searched on the net and found this chart:

power-consumption-vs-clock-speed

How can that horizontal line at the top left happening?

Nhu Thai Sanh Nguyen

Posted 2017-07-10T02:33:12.007

Reputation: 517

I think crypto mining is a topic better suited for the crypto stack exchange.. – None – 2017-07-10T02:45:53.043

Logic says you are either not limited by the GPU's clock or some of your measurements are wrong. – Wesley Lee – 2017-07-10T03:21:32.010

Some research shows mining hash rate scales with a proportion of RAM and core clock, the second being the limitation in some cases. This would explain your case. If this is the explanation then it seems off topic. – Wesley Lee – 2017-07-10T03:24:43.310

@NickAlexeev, I'm mining Ether Classic. – Nhu Thai Sanh Nguyen – 2017-07-10T04:43:39.200

@WesleyLee, from what I know, hash rate doesn't scale with RAM at all. As long as there's enough memory to load the mining client, it's all good. 2 same GPU with 4Gb and 8Gb memory show the same hash rate. In my case, core clock doesn't drop either despite a reduction in power draw. :| – Nhu Thai Sanh Nguyen – 2017-07-10T04:50:43.480

Answers

2

Ethereum mining is entirely memory bound rather than CPU bound.

What that means is that the stream processors, while necessary for some caclulations, are generally not the bottleneck. Memory bandwidth is.

Because of this overclocking the RAM on the graphics card is what will increase performance, not the GPU itself. You can underclock the GPU or restrict the power to the GPU, to a point, and see only minor drop in hash rate. This can give you a performance boost and power saving at the same time.

Read more at Small tweaks to improve hash rate and reduce power.

From the Ethereum github page:

Ethash PoW is memory hard, making it basically ASIC resistant.

...

It is designed to hash a fast verifiability time within a slow CPU-only environment, yet provide vast speed-ups for mining when provided with a large amount of memory with high-bandwidth.

This is simply rephrasing what I've said. You don't need a massive amount of CPU power but having a large quantity of high bandwidth memory, as you would find on a graphics card, is what will really be the key factor for hash rate.

Mokubai

Posted 2017-07-10T02:33:12.007

Reputation: 64 434

It is probably turning off stream processors then, but either way it is restricting processing performance. – Mokubai – 2017-07-10T05:52:27.847

Asked: 2017-07-10T02:33:12.007

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Active: 2017-07-10T09:24:00.507