All About BTC, LTC, ETH mining as well as other alternative crypto currencies
We have already published some interesting findings about the power usage of the new GeForce GTX 750 Ti graphics cards when used for crypto mining and recently we have built a 6-card GTX 750 Ti mining rig. We have taken some time to also measure the power usage of the individual cards as well as the total power usage of the whole system in order to give you some additional details about what you can expect in terms of power consumption from such a mining rig. We have used Gigabyte GeForce GTX 750 Ti video cards (N75TOC-2GI) that do have an external onboard PCI-E power connector and we started by measuring the total power that goes to a single video card both trough the PCI-E slot as well as through the external PCI-E power connector with the help of a watt meter that is attached directly to the power lines going to the video card (using a powered extender).
As you can see on the photo the total power used by the video card is about 31W (with the card running at +135 MHz for the GPU and + 610 MHz for the video memory). Note that this power measurement is on the 12V line from the PSU going to the video card, so it does not take into account the power efficiency of the power supply and as a result the total power used by the video card from the mains will be higher by something like 15-20% (depending on the PSU). Note that we have used a powered PCI-E extender with USB 3.0 cable for the data lanes. This extender takes all the power provided to the PCI-E slot of the video card through a 4-pin molex connector and supplies both the 12V and the 3.3V power that the card uses drawing only power from the 12V line (there is a voltage regulator to output 3.3V from the 12V input on the extender’s board). So what the wat tmeter shows is the total power going from the PSU to the video card and in the case of the Gigabyte GTX 750 Ti it was 31W. Again the total power usage from the mains will be higher as this measurement does not take into account the power efficiency when converting 220V/110V to 12V.
So what is the situation with the total power usage per GTX 750 Ti video card from the mains? The easiest way to check that is to measure using a watt meter connected between a power socket and the power supply of the computer the total power consumption of the system with 6 cards and then disconnect one card and to measure again. The difference we got using this method was about 79W, though this is not for the video card only as it also affects a bit the overall power consumption of the whole system. Also note that the measured 374 Watt for the 5-card and 453 Watts with the 6-card setup are with the video cards overclocked to +135 MHz/+610 MHz. The results we’ve got with the cards running at the stock frequency were 367W with 5-cards and 432W with 6 cards or a 65W difference per card (total power used from the mains). This difference of 14 watts between stock and overclocked frequencies brings roughly about 40 KHS more in terms of hashrate for Scrypt mining (per card). Have in mind that our power supply used had an efficiency rating of about 80-85%, so this means that 15-20% of the total power used at the mains is actually wasted in the conversion between 220V/110V and 12V.
Another interesting thing that we have noticed while testing the power usage and overclocking capabilities of the 6-card GeForce GTX 750 Ti mining rig was the total power consumption that we got for the system with the power target limit changed from the standard setting of 38.5W to the 65.5W. The watt meter showed an increase of power from the 453W with the 38.5W power target limit to 556W with the 65.5W power limit – about 100W increase with the same operating frequencies with a slight increase in performance of about 90 KHS total from the 6 cards or roughly a bit over 1W per KHS and in our opinion this is not worth the extra increase in power usage, so raising the power limit may not always be a good idea!
Today we have built a 6-card mining rig using Gigabyte GeForce GTX 750 Ti (N75TOC-2GI) video cards that are based on the latest Nvidia Maxwell architecture promising very good performance for mining with low power usage. Our own initial tests of the GeForce GTX 750 Ti as a single card for mining have shown very promising results as well, so we’ve decided to see what we can expect from a mining rig and put together 6 cards with PCI-E x1-x1 extenders on an AsRock H81 Pro BTC motherboard with Intel Celeron G1820 CPU and 4GB of RAM running Windows 7.
The result we’ve got from the 6-card mining rig for mining Scrypt with CUDAminer was a total of 1480 KHS as hashrate. Then after overclocking the video cards to the maximum stable result we managed to get (+135 MHz for the GPU and +610 MHz for the video memory) we’ve managed to increase the total hashrate to about 1615 KHS. We went as far as to increase the power target limit from the default 38.5W by modifying the video BIOS of the cards. With the modified video BIOS we have managed to get slightly more than 1700 KHS with a lot of extra power used by the whole system that made it not worth it the so little extra increase in the hashrate. Just to be sure that the x1-x1 PCI-E risers (not powered) might be the cause of slight performance drop we’ve replaced them with x1-x16 USB 3.0 powered extenders, though that did not change the performance we got from the cards.
The Gigabyte GeForce GTX 750 Ti video cards we used for the mining rig do have an external PCI-E power connector, however it seems that unlike AMD graphics with OpenCL, when using Nvidia-based GPUs with CUDA for mining the use of a x1 PCI-E lane to access the video card does introduce slight performance drop and if you multiply the 10-15 KHS less per card for a 6-card mining rig it is not so little. So it is important to know that if you are going to build a GTX 750 Ti-based mining rig you will be getting slightly lower hashrate if you are using PCI-E extenders as compared to what you will get with cards inserted in x16 PCI-E slot directly. Also there is some variation between cards in terms of the maximum overclock supported that results in different maximum frequencies that you can achieve, for example if one card is able to do +135/+700 MHz for the GPU/VRAM the second one could be maximum +100/+600. This means that in a 6-card mining rig you will need to either sync all of the cards and use the same lower settings for overclocking the GPUs to ensure they will run stable or to not have the settings synced and find the maximum for each of the cards.
Another interesting thing we have noticed is that while the use of T5x24 kernel for a single card with CUDAminer for best results, for a 6-card mining rig the use of T10x24 might sometimes provide slightly higher hashrate than T5x24, so you should try with both and see what works better in your individual case. Tomorrow we are going to be doing some more testing of the 6-card GTX 750 Ti mining rig that will be focused mostly on the power consumption as this is also a very important thing when talking about crypto currency mining.
Nvidia is advertising their new GeForce GTX 750 Ti GPUs based on the new more power efficient Maxwell architecture as being with 60W TDP, but in truth they seem to be much more power efficient than that. A lot of people are already interested in using these new GPUs from NVidia for mining, because the performance that the Maxwell delivers seems to be very good in terms of hashrate per Watt ratio. When you use CUDAminer to mine with the GTX 750 Ti you will notice the card will quickly reach the 100% power target limit and normally you are not allowed to increase the power limiter of the card above the 100% value, you are allowed to only lower it further. The interesting thing here however is that the default TDP limit for GTX 750 Ti is actually set to 38.5W inside the BIOS and the minimum of 78% you can go down to is equal to just 30W TDP and as we’ve mentioned Nvidia is talking about 60W TDP for these cards. The fact that the cards are actually limited to 38.5W by their power limiter is something that can prevent you from squeezing the maximum performance that you can get by overclocking the GPU and/or the video memory of the card, so increasing the TDP limit by modifying the BIOS and reflashing the video card with the modified BIOS can help you with that.
Since many of the GeForce GTX 750 Ti video cards do not have external PCI-E power connector you will be limited to the maximum power that these cards will be able to use due to the 75W maximum of power that PCI-E x16 slot can provide by specifications (66W for the 12V line that you will be actually using, the other ~10W are for the 3.3V line). But even this will provide more than enough headroom since the default TDP limit is not 60W as we though, but really just 38.5W, so lots of headroom for overclocking. In order to be able to modify the BIOS of your video card you will need to first save the original one from the card, you can use GPU-Z for that and make sure you keep the original BIOS as a backup and save the modified one as a separate file (you can also backup the BIOS with the nvflash under DOS if you are having trouble with GPU-Z). Then you need to fire up Kepler Bios Tweaker and open up your BIOS file and edit is as shown in the screenshot above (left is original, right is the modified) in order to get 65.5W as the maximum TDP of the card. After that you need to flash the BIOS back to your card using the provided nvflash, you can download the flasher and the Kepler Bios Tweaker along with a standard BIOS from a reference 750 Ti and a modified version of that BIOS to allow 65.5W TDP from the link below. We suggest that you save the BIOS from your own video card and modify it, also do have in mind that modifying and flashing modified video BIOS to your video card can be dangerous, so you should be extra careful what you are doing and not modifying things that you should not or does not know what they are for!
The procedure described above will work for increasing the power target limit on other video cards as well not only on GTX 750 Ti, however before increasing the limit make sure that your video card cooling can keep the card cool enough. Using the video card fro mining will bring the power target to 100% in most cases even without overclocking the card additionally, so just by increasing it you might e able to squeeze some extra performance even without overclocking it further. The version of nvflash provided in the archive below is the latest one that will work with the GTX 750 Ti as well as with older video cards, it is the DOS version of the flasher as the Windows version of nvlfash does not seem to work properly – it does not want to flash the modified BIOS to the card saying that the BIOS digital signature is wrong. No problems flashing the modified video BIOS though trough the DOS version of nvflash 5.163, so we have only included the DOS version that will work with the method described above. There are two BAT files configured to flash the modified video BIOS and to restore the original version of the included reference design board BIOSes, you just need to run “nvflash your_bios.rom” and confirm with “y” when asked by nvflash (make sure you’ve made a backup of your original video BIOS first!). Again, be aware that video BIOS flashing and modification can be dangerous and can temporary render your video card useless, at least until you reflash it with the original BIOS, so do keep a backup of the original! Also note that increasing the TDP limit beyond the recommended value above could also be dangerous, so be well aware that this modification can be dangerous! Feel free to share your results for overclocking and mining hashrate after increasing the TDP limit of your GTX 750 Ti or another board in the comments below.