Apple MacBook Pro 13in: where's the Turbo Boost?

10 Mar 2011

The Apple MacBook Pro 13in is a glorious laptop. It's thin and light, gorgeous both to look at and to use, and it packs no small amount of power in its tiny chassis. Yet our tests have uncovered a performance issue that will affect every user.

We ran our new Real World Benchmarks on the top-end model, with a dual-core 2.7GHz Intel Core i7-2620M processor, 4GB of DDR3 and a 500GB hard disk. It's a very fast laptop for its size, as a final score of 0.70 shows - that's only around 20% slower than the top-end quad-core 17in model. Yet it's not quite as fast as it should be.

We first noticed a problem when the benchmarks finished five full runs and the results popped up on screen: the times taken to complete several of the most intensive tests were rising with each run. This would suggest an overheating problem, so we ran a temperature monitor to find out how hot this Sandy Bridge CPU was getting. Here are the readings both when idle and under full load:

Temperatures

We should point out that 93°C is not necessarily too high for a modern CPU, but it is the root cause of the bigger performance problem. To explain, here's that full-load temperature again, along with Intel's own Turbo Boost monitor:

Turbo 13in

The temperature of 93°C was reached with the processor peaking at 2.7GHz. The Core i7-2620M should be able to Turbo Boost up to a maximum of 3.4GHz, but in this laptop it doesn't top 2.7GHz at any temperature.

For comparison, here is the same reading from the Core i7-2720QM in the top-end 17in model:

Turbo 17in

Here it's being Turbo Boosted from its stock 2.2GHz. Notice how the blisteringly fast 17in model only peaks a few degrees hotter than the 13in, and that's with the maximum boost. As soon as the fans kicked in that settled comfortably in the high eighties.

What does this all mean? Well, if the CPU in the MacBook Pro 13in hits 93°C at stock speeds, we can only imagine how hot it would get if Turbo Boost was allowed to kick in. So we suspect Apple has disabled it completely to prevent overheating in such a tiny chassis.

It's an issue we haven't seen highlighted, perhaps because to the end user it doesn't really affect the day-to-day experience - and it shouldn't put you off buying what is in every other way a fantastic piece of kit. But Apple is promoting this on its website as a Turbo Boost-enabled laptop. We've asked Apple for comment and await its response.

UPDATE 11/3/11, 13:30: First, we must just clarify, it's definitely not that the processor is dynamically choosing not to apply Turbo Boost due to the temperature under load; we've used this MacBook Pro for a week now and the Intel Turbo Boost monitor doesn't report a boost at all, whatever the temperature and task.

We also now have the £999 model with its 2.3GHz Core i5 processor in the Labs. We ran the same tests on that one and can confirm that its processor is Turbo Boosting in Boot Camp as it should. The issue is unique to the i7 model.

However, secondly, we must doff our caps to Anandtech and show you our reading from a utility they used called MSR Tools:

We stand corrected on one count: it is indeed Turbo Boosting in OS X. We ran a temperature monitor for several minutes as well, and those boosted speeds occurred with a peak temperature of 93°C, the exact same as the peak in Windows without the boost.

The Turbo Boost issue therefore looks to be one of processor cooling in Windows. That affects a far smaller group of users that an OS X flaw, but it remains a mystery: does the Core i7 model of the MacBook Pro 13in run so hot under Windows drivers that Apple has chosen to disable Turbo Boost? Our tests make that a plausible scenario.

Heat is an issue that's difficult to ignore. Even on the Core i5 model, just unpacking a large zip file had the Turbo Boosted Core i5 quickly rising to 90°C; during a stress test that hit 99°C. And that has a knock-on effect: with an IR temperature gun we measured the aluminium underside of the laptop at a thigh-scalding 60°C!

We're more sure than ever that Apple had a real task on its hands getting such fast processors into this chassis, and from everything we've seen we're leaning away from the top-end 13in model as a purchase. If the sight of our benchmarks gradually getting slower with each consecutive run wasn't enough to highlight the cooling problem, a heat gun pointed at the metal underside certainly was.

There is one plus to the Turbo Boost mystery, however: if you intended to buy a MacBook Pro 13in and install Windows on it, you'll probably find the cheaper model actually runs faster than the top-end one. Save yourself £300.

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