One of the features both AMD and Intel have adopted in the past five years is the idea of a Turbo Mode (Intel) and Turbo Core (AMD). The CPUs of both companies will raise the operating frequency if there’s thermal headroom to do so, then lower it again if the thermal load is too high. Most of the time, this feature is a positive — unless you happen to be using AMD’s latestKaveri APU, which has a problem that can significantly lower its overall performance under certain specific conditions. AMD is aware of the problem — we’ve been working with them for over a week — and is doing their own full investigation.
We were first alerted to a potential issue by reader Daniel, who showed us evidence of an odd behavior when Kaveri was running in APU-only mode (meaning without a discrete graphics card). When an APU’s integrated GPU is put under load, the maximum CPU clock speed can drop sharply, from 3.7GHz to 3.0GHz. This occurs regardless of thermal load, as shown in the screenshot below, where I’m running Cinebench and Litecoin mining at the same time.
We took this information to AMD and have worked extensively with the company over the past week to better understand the behavior.
The good news: Kaveri’s clock drop has a negligible impact on APU-only gaming
The A10-7850K’s clock speed drop is intentional, and it’s meant to give the graphics core more frequency headroom. AMD’s APM (Advanced Power Management) ability detects when specific GPU workloads are running, and cuts the CPU frequency to ensure plenty of thermal headroom is available to the GPU. AMD’s APM and CPB (Core Performance Boost) settings can both be overridden in BIOS, to ensure that the CPU maintains a steady clock speed.
The CPU clock pulldown does not impact APU gaming performance in any title we tested. We tested multiple games, including Skyrim, Orcs Must Die 2, Torchlight 2, Civilization V, and Metro: Last Light using only the APU’s integrated graphics. None of these games exhibited performance degradation of any kind when running solely in APU mode. In fact, Skyrim ran slightly faster with Turbo Core enabled as opposed to disabled when we tested the integrated GPU.
The bad news: Dual Graphics takes a whallop
We were actually working on an evaluation of Kaveri’s Dual Graphics performance when we first heard about this issue. While the clock drop doesn’t seem to impact AMD’s APU performance, the same can’t be said of Dual Graphics configurations. The data below is culled from Metro: Last Light in 1600×900, Medium Detail, with 4x AF enabled.
Metro: Last Light – Turbo Core Off (Left) versus On (Right)
The black “scribble” on the right-hand image is the result of tremendous variation in each frame’s rendering performance. What you’re seeing there is the result of erratic delivery and a very noticeable stutter. The impact on 1% frame times — the “worst” frame times rendered in the scene — is significant. Here’s the data from the same Metro: Last Light runs as shown above, but this time with the 1% and 0.1% — the two worst cutoffs for frames — broken out in particular.
Average 1% frame times are 39% worse when Turbo Core is enabled — 14 FPS versus 20 FPS. The real-world impact of that is that the game feels extremely jerky when running in Dual Graphics mode. The problem isn’t caused by running at the slower clock speed — we retested the game at a flat 3GHz (with Turbo Core disabled) and saw nothing like the 3.7GHz pattern. The issue here has something to do with the Turbo Mode pulldown that the GPU enforces, even if thermals are fine.
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