Analysis and Optimization of Dynamic Voltage and Frequency Scaling for AVX Workloads Using a Software-Based Reimplementation

This repository contains the LaTeX sources of my bachelor's thesis (conducted at KIT between 05/03/2019 and 09/02/2019), all source code written for the research done in the thesis, and CSV sheets with the raw results.

Abstract

While using the Advanced Vector Extensions (AVX) on current Intel x86 processors allows for great performance improvements in programs that can be parallelized by using vectorization, many heterogeneous workloads that use both vector and scalar instructions expose degraded throughput when making use of AVX2 or AVX-512. This effect is caused by processor frequency reductions that are required to maintain system stability while executing AVX code. Due to the delays incurred by frequency switches, reduced clock speeds are retained for some additional time after the last demanding instruction has retired, causing code in scalar phases directly following AVX phases to be executed at a slower rate than theoretically possible.

We present an analysis of the precise frequency switching behavior of an Intel Syklake (Server) CPU when AVX instructions are used. Based on the obtained results, we propose AVXFreq, a software reimplementation of the AVX frequency selection mechanism. AVXFreq is designed to enable us to devise and evaluate alternative algorithms that govern the processor’s frequency smarter with regard to workloads that mix both AVX and scalar instructions. Using an oracle mechanism built atop AVXFreq, we show that the performance of scalar phases in heterogeneous workloads can theoretically be improved by up to 15 % with more intelligent reclocking mechanisms.

Build & Run

Note: All code was specifically written to run on an Intel Core i9-7940X. Both AVXFreq and the analysis framework are theoretically capable of running on other Skylake (Server) processors as well, however, minor modifications will be necessary.

AVXFreq

• Grab a fresh copy of the Linux 5.1.0 source code
• Apply avxfreq.diff
• make && sudo make modules_install install
• Ensure the following settings are configured in your motherboard's UEFI:
  • SpeedShift (HWP/CPPC): Off
  • SpeedStep (EIST): On
  • Turbo Boost: On
  • Turbo License Level Offsets 1 and 2 both set to 1
• Boot your kernel with intel_pstate=avxfreq

Reclocking Analysis Tool

• Build and run a kernel with AVXFreq and reclocking-analysis/entry.diff applied
• Build the kernel module:
  • cd reclocking-analysis/kernel
  • make -C <kernel source path> M=$(pwd) && sudo make -C <kernel source path> M=$(pwd) modules_install
• Build the user-space component:
  • cd reclocking-analysis/user
  • make
• sudo php reclocking-analysis/user/run-all.php to run all tests (this requires PHP 7.2)

The analysis framework will automatically detect whether the system is running with HWP or AVXFreq and adapt accordingly.

License

You may redistribute, present, build, and run everything in this repository, as long as you keep/show a clear notice of authorship. All modifications and other uses require explicit written permission by me.

Execute this software at your own risk. I am not responsible for any damage done to your or anyone's computer.