Over the weekend, several websites have relayed a long thread on the Apple discussion forums about a problem occurring on the 15 and 17" models of the 2011 MacBook Pro.
A Wiki has even been created to describe the problems. To describe them briefly, the laptop freezes, preventing any access through the keyboard or trackpad. The problem would occur even if the laptop is cold, but only when an heavy workload is launched on the CPU and the AMD GPU, and would not occur while using the integrated Intel chip.
Since we have a 15" model, we have run numerous tests in order to reproduce the problem, while monitoring with the very good Hardware monitor software, which allows to monitor very precisely all the sensors of the laptop.
Before jumping to any conclusion, here are a few things that we noticed:
- With a very light workload, the CPU (a 2.3 GHz quad-core) consumes between 2.5 and 5 W. Working at full capacity, it reaches 45 W.
- When the integrated Intel video chip is used, the AMD chip and its RAM consume 0.6 W. When activated, it consumes 5.5 W and goes up to 20 W at full capacity.
- The fans spin at about 2,000 rpm on light work load, and can go up to 6,200 rpm on an heavy workload.
Interestingly, we have noticed that they don't accelerate immediately when the workload intensifies. About 20 seconds past before the start accelerating. It is probably the time it takes to the laptop to heat up, but also a choice made by Apple to prevent them from accelerating an stopping all the time like they do on the PowerMac G5.
- During our tests, the CPU reached 94ºC (201ºF) without any problem or slowing down. The temperature never went over that, even when we pushed the laptop harder.
To run the test, we used 3 softwares in addition to Hardware monitor, gfxCardStatus to force the switch from one graphics solution to the other, PowerFractal to push the CPU at its full capacity and Cinebench to do the same with the GPU.
Despite our numerous tries, we were able to reproduce the problem only once, but it is interesting to see that it occurred only when we tried for the first time, when the laptop was still cold. All the other tests only made the fans spin faster.
It is still too early to make any definitive conclusion, but it would seem that the freezing, definitely caused by the graphics card is due to the inertia of the fans during the first 20 seconds. The massive workload applied to the GPU, 20 W, added to the 45 W of the CPU seems to prevent the first one to evacuate the heat an causes the freezing.
To back this explanation, here is a picture from iFixit showing that the CPU and the GPU share the same heat sink:
On this picture, the CPU (rectangle) is just under the GPU (square). The heat of the CPU therefore has to go over the GPU to get out. If the fans are not spinning fast enough, the CPU will heat the GPU up, and the latter is not reacting well when its temperature is increasing too much.
We can wonder what Apple will do to solve the problem. It is unlikely that they decide to lower the frequency of the Radeon chip. It would have little consequences on the global TDP of the two chips. The solution might be more simple as we believe that starting the fans earlier when the Radeon starts working would prevent it from overheating, and it can be done easily through an SMC update.
In the mean time, all users who are confronted to the problem can either prevent the Radeon and the CPU from working at full load, or let the laptop get warm before launching any heavy workload.