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x86-64 micro-architecture update

Summary

Fedora currently uses the original K8 micro-architecture (without 3DNow! and other AMD-specific parts) as the baseline for its x86_64 architecture. This baseline dates back to 2003 and has not been updated since. As a result, performance of Fedora is not as good as it could be on current CPUs.

This change to update the micro-architecture level for the architecture to something more recent.

Owner

Current status

Detailed Description

After preliminary discussions with CPU vendors, we propose AVX2 as the new baseline. AVX2 support was introduced into CPUs from 2013 to 2015. See CPUs with AVX2.

Along with AVX2, it makes sense to enable certain other CPU features which are not strictly implied by AVX2, such as CMPXCHG16B, FMA, and earlier vector extensions such as SSE 4.2. Details are still being worked out.

A test rebuild of a distribution largely based on Fedora 28 showed that there is only a small number of build failures due to the baseline switch. Very few packages are confused about the availability of the CMPXCHG16B instruction, leading to linking failures related to -latomic, and there are some hard-coded floating point results that could change due to vectorization. (The latter is within bounds of the usual cross-architecture variation for such tests.)

Benefit to Fedora

Fedora will use current CPUs more efficiently, increasing performance and reducing power consumption.

Moreover, when Fedora is advertised as a distribution by a compute service provider, users can be certain that their AVX2-optimized software will run in this environment.

Scope

  • Proposal owners: Update the gcc and redhat-rpm-config package to implement the new compiler flags. It is expected that the new baseline will be implemented in a new GCC -march= option for convenience.
  • Other developers: Other developers may have to adjust test suites which expect exact floating point results, and correct linking with libatomic. They will also have to upgrade their x86-64 machines to something that can execute AVX2 instructions.
  • Release engineering: #8513
    • All Fedora builders need to be AVX2-capable.
    • Infrastructure ticket: #7968
  • Policies and guidelines: No guidelines need to be changed.
  • Trademark approval: N/A (not needed for this Change)

Upgrade/compatibility impact

Fedora installations on systems with CPUs which are not able to execute AVX2 instructions will not be able to upgrade.

How To Test

General system testing will provide test coverage for this change.

User Experience

User should observe improved performance and, likely, battery life. Developers will benefit from the knowledge that code with AVX2 optimizations will run wherever Fedora runs.

Dependencies

There are no direct dependencies on this change at this time.

Contingency Plan

It is possible to not implement this change, or implement a smaller subset of it (adopting the CMPXCHG16B instruction only, for example).

  • Contingency mechanism: Mass rebuild with different/previous compiler flags.
  • Contingency deadline: Final mass rebuild.
  • Blocks release? No.
  • Blocks product? No.

Documentation

The new micro-architecture baseline and the resulting requirements need to be documented.

Release Notes

Release notes must mention how users can determine whether their system supports AVX2 prior to upgrading, for example by running grep avx2 /proc/cpuinfo.

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