In our initial review of the two 6th Generation Intel Skylake-K processors launched on August 5th, the i7-6700K and the i5-6600K, our comparative analysis to the previous generations of Intel processors was for the most part, positive. On the whole, clock-for-clock performance was a marginal increase over previous generations but the cumulative end-to-end effort of several generations of upgrades, plus for those that overclock, gave a substantial reason for those in CPU limited workloads to find an upgrade (along with benefits on the chipset and DRAM side as well). However, one element of the equation was puzzling at the time – the performance of games using discrete graphics cards was marginally lower with the new platform compared to older platforms when looking at average frame rates.
During our testing, it is not uncommon to see two platforms that perform similarly to have a reasonable margin of error, often ±1%, due to variations in pre-initialised cache structures, or in the case of games like GRID that rely on a random sequence to provide the end-result numbers. Despite this, we noticed that for Skylake-K we saw consistent drop in our discrete GPU testing, often around the -1% to -3% mark but sometimes as low as -5% or -7% when we compared it to both Intel’s 5th Generation (Broadwell) and 4th Generation (Haswell). Other websites such as The Tech Report also noted these results, placing Broadwell’s numbers at the top of the stack (if only marginal). Some commentary at the time focused on Broadwell’s use of eDRAM in the desktop components which can aid performance while retaining a frequency deficit, although given our analysis of the eDRAM in Broadwell as a victim cache rather than a transparent DRAM cache it seems less likely that this is the case, plus we also now have new information coming post launch about this issue. But if we remove Broadwell as a special case, it was still concerning that the i7-6700K lagged behind the i7-4770K despite being higher in frequency and clock-for-clock performance.
Before it came time to publish our Skylake review, we performed our initial analysis and ended up with our results. Whenever the results are worse than expected, we typically discuss with the manufacturer regarding any anomalies and if they can account for them (or something doesn’t seem to be configured properly). So we passed on our data to Intel as well as ASUS due to our setup at the time, and did not hear anything back for a number of weeks except the odd whisper of ‘we are looking in to it’. Then, in our meeting with Intel at the Intel Developer Forum in mid-August, an Intel processor engineer said that they were still working on it internally, but from their testing it seems that one of the registers controlling an internal frequency was not being set properly during start-up – as in not being set to Intel’s recommended value.
Another couple of weeks later, we were contacted by ASUS who shed a lot more light on the issue. The register in question is called the FCLK (or ‘f-clock’
, which controls some of the cross-frequency compensation mechanisms between the ring interconnect of the CPU, the System Agent, and the PEG (PCI Express Graphics). Basically this means it is to do with data from the processor to the GPUs. So when data is handed from one end to another, this element of the processor manages the data buffers to allow that cross boundary migration in a lossless way. This is a ratio frequency setting which is tied directly to the base frequency of the processor (the BCLK, typically 100 MHz), and can be set at 4x, 8x or 10x for 400 MHz, 800 MHz or 1000 MHz respectively.
FCLK is in the top left, between the CPU and the PCIe lanes.
The default value of the FCLK is at 800 MHz for both mobile and desktop Skylake processors, and it is this value that all the motherboard manufacturers have validated their systems on – such as overclocking and margins due to external environmental factors. However, the Intel recommended value for desktops, as dictated in their ‘tuning guide’ for motherboard manufacturers was 1000 MHz, or the 10x ratio setting. The recommended value for laptops is still the 8x ratio setting. [...]
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