Intel's 10nm Cannon Lake and Core i3-8121U Deep Dive Review

CPU Performance: Memory and Power

Memory

With the same memory setup as the standard Skylake-style core, without any other improvements, we should expect Cannon Lake to perform exactly similar. Here’s a recap of the memory configuration:

For this test we put our Cannon Lake and Kaby Lake processors through our Memory Latency Checker test. Both systems had turbo modes disabled, forcing them to run at 2.2 GHz for parity and a direct microarchitecture comparison.

In this instance we see very little different between the two through the caches of the chip. Up to about 2MiB, both chips perform practically identical. Beyond this however, going out to main memory, there is a big discrepancy between the Kaby Lake and the Cannon Lake processor. In essence, accessing Cannon Lake main memory has an additional 50% latency.

At first we thought this was a bug. For both systems to have dual channel memory and running DDR4-2400, something had to be wrong. We double checked the setups – both systems were running in dual channel mode, giving the same memory bandwidth. The Cannon Lake processor was running at DDR4-2400 17-17-17, whereas the Kaby Lake system was at DDR4-2400 16-16-16 (due to memory SPD differences), which isn’t a big enough change to have such a big difference. The only reason we can come up with is that the memory controller on Cannon Lake must have additional overhead from the core to the memory controller – either a slower than expected PLL or something.

Power

Measuring the power of these thermally limited systems is somewhat frustrating in that we are at the whims of the quality of silicon at play. Bad silicon can get hotter faster, causing thermal throttling, or it depends on how the systems are set up for the peak power numbers. We use AIDA’s power monitoring tool to give us the power numbers for the CPUs during our POV-Ray test.

If you saw our How Intel Measures TDP article, you may be familiar with the concept of Power Limits. Intel processors have several power limits set in firmware – these values are set by the system manufacturer, and Intel provides ‘guidelines’. The two key values are Power Limit 1 (PL1), which describes a steady state scenario, and Power Limit 2 (PL2) which describes a turbo scenario. The reason why the system manufacturer gets control of these is because they might put a high wattage processor into a small form factor system, and need to manage performance – this is why we sometimes see a Core i7 be outperformed by a Core i5 in the same system design.

In most cases, PL1 is set to the TDP of the processor – in this case, it should be 15W. As shown on the graph above, this is true for our Core i3-8130U system, which shows a steady state power consumption of 15.0W. The Cannon Lake processor however only peaks at 12.6W.

PL2, the turbo power of the processor, can also be determined. For the Kaby Lake processor, we see a peak of 24.2 W, while the Cannon Lake has a turbo only of 18.7 W. We see that the turbo time period for the Cannon Lake processor is also much shorter, leading to a PL1 / steady state power much sooner.

Part of this is because we are pitting the Cannon Lake laptop against the Kaby Lake mini-PC. That being said, the Cannon Lake laptop is a beefy 15.6-inch unit, and has cooling for both the CPU and the discrete RX540 graphics, which isn’t doing much during this test except showing a basic 2D display.

That is arguably a lot more cooling than the mini-PC provides, plus we are cooling it with additional fans. These settings are more a function of both Lenovo doing what it wants to in firmware that we can’t change, but also the chip as engineered.

We can also calculate some level of efficiency here. By taking the area under the graph and correcting for the base line power (2.9 W on KBL, 3.6 W on CNL), we can calculate the total power consumed during the test. We get the following:

• Core i3-8121U (CNL) consumes 867 mWh
• Core i3-8130U (KBL) consumes 768 mWh

So this means that Cannon Lake is slower in AVX2 frequency, consumes more power, and scores 25% less in POV-Ray (see previous pages). That’s damning for the design.

To further look into this data, we disabled the turbo modes on both processors and ran our POV-Ray workload adjusting the number of threads being loaded and then measured the power consumption. In this scenario we are running at the same frequency, measuring the steady state power, and are thus at the whims of both the voltage set on both processors and the efficiency. Our tool also lets us measure the power of just the cores, so we’ll plot core power against threads loaded.

If this graph was the definitive graph, it shows that Cannon Lake is vastly inefficient (at 2.2 GHz) compared to Kaby Lake.