Intel's Thread Director Coming to Linux 5.18 to Fix Alder Lake Performance Issues

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An Alder Lake performance uplift is coming closer

The upcoming Linux version 5.18 will improve the performance of Intel's latest 12th Generation Core 'Alder Lake' processors, as it will come with the company's new drivers that optimize usage of (P)erformance and (E)fficiency cores. These CPUs rank as many of the best CPUs for gaming and other tasks, but at present, they work best on Windows 11 PCs, which is typically unheard of when compared to performance in Linux.

Intel first published patches that enable HFI on Linux late last year, but the first version of Linux to integrate these new drivers will be 5.18, which is due in Spring 2022. The new version of Linux will assign background and less important workloads to energy-efficient cores, thus improving the performance of Intel's newest CPUs, reports Phoronix. Unfortunately, the release date for Linux 5.18 is unknown.

Unlike Windows 11, Linux currently doesn't have the appropriate support for Intel's Thread Director technology that uses the Enhanced Hardware Feedback Interface (HFI). HFI allows the OS to properly allocate threads to the high-performance Golden Cove and energy-efficient Gracemont cores, which is why Intel's hybrid Alder Lake CPUs perform better under Windows.

Without HFI support, the Linux kernel makes decisions on whether to use P or E cores using Intel's ITMT/Turbo Boost Max 3.0 driver that relies on the information exposed by the firmware. That's why in many cases it prefers the fastest cores with the highest frequency (i.e., Golden Cove cores) and does not use Gracemont cores even when possible, which leads to their underutilization.

By contrast, Intel's Thread Director communicates actual numeric performance and numeric power efficiency values of each CPU core in a 0 – 255 range to the OS. If either the performance or energy efficiency capability of a CPU core is 0, the hardware dynamically adapts to the current instruction mix and recommends not assigning any tasks on this core for performance, energy efficiency, or thermal reasons.

Significant changes to scheduling in an operating system can have wide-ranging effects, and it can take time to work out all the kinks. Intel's Thread Director can help provide feedback to the OS on where threads will run best, but there's rarely a single 'best' answer and tuning can take time. We'll be to see just how much benefit the new scheduler brings to Alder Lake chips in the next couple of months.
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