Connecting two external monitors to my Retina Macbook Pro 2012 has no problem except for more fan noise, which is a little bit annoying since with one external monitor one doesn't hear the noise at all.

I'm wondering what's the cause of this? What is the bottleneck here? Is it because GPU Clock Speed or Memory of Graphics Card?

rMBP 2012 has Nvidia GeForce GT 650M with 1 GB GDDR5. If I switch to rMBP 2013/2014 which has Nvidia GeForce GT 750M with 2 GB GDDR5 and say if the problem disappears (I'm just guessing), is it mainly because of higher GPU speed or more memory?

Btw, 650M scores 1278 while 750M scores 1461.

Update 01/25/15: iStat Menu: iStat Menu monitors both CPU/GPU usage and temperature. Compared to only one external monitor, two external monitors causes CPU & GPU temperature up by 20F higher. However, CPU & GPU usage & GPU memory usage all remain the same.

The bottom line is that iStat Menu can't explain the increase in CPU/GPU temperature.

  • Did you look in your Activity Monitor ? – Ruskes Dec 6 '14 at 16:48
  • Yes, I did. It shows that CPU usage is pretty low as always, but it doesn't show any GPU-related info. – Titanic Dec 6 '14 at 17:04
  • try smcFanControl to see what temps the GPU is running at & fans speeds. – Tetsujin Dec 6 '14 at 17:06
  • use the Intel made GPU check, it is made by Intel so it would be accurate. Intel Power gadget. software.intel.com/en-us/articles/intel-power-gadget-20 – Ruskes Dec 6 '14 at 17:07
  • smcFanControl gives you the temperature and fan speed, but it still won't let me know what's the cause of this. Is it GPU Speed or Memory Usage. – Titanic Dec 6 '14 at 17:12

I would recommend the iStat Monitor to find out what is going on.

In comparison the 750 vs 650:

50/45 watt power consumption

and 967 vs 900 MHz clock

would not explain the higher heat generation.

However due to the RAM size difference it could explain that the 650 has to work harder (at max power) resulting in higher heat generation, while the 750 does not break the sweat with 2 monitors.

  • I do think iStat Menus would work. However, I installed iStat Menus on my computer before and the trial period has passed. So it won't work for me now. I'm hoping to get some theoretical answers: e.g. what would happen if the GPU Speed is the bottleneck and what would happen if the memory is the bottleneck. – Titanic Dec 7 '14 at 21:07
  • So you are not looking at what is the temperature but the data load/flow! – Ruskes Dec 7 '14 at 21:20
  • I am looking into the causes of a high temperature. If iStat Menus tell me that my GPU runs at 200 degrees, I know that my GPU is running at its limit, but I still don't know what's the cause of it: GPU Speed or Memory. – Titanic Dec 8 '14 at 0:25
  • I do agree that iStat Menus is one solution, since it can tell me the usage of GPU and memory of Graphics Card. – Titanic Dec 8 '14 at 0:26

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