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List:       fedora-devel-list
Subject:    Re: Fedora 32 System-Wide Change proposal: x86-64 micro-architecture update
From:       Igor Gnatenko <ignatenkobrain () fedoraproject ! org>
Date:       2019-07-23 2:31:52
Message-ID: CAFMg4WBixqh7doAnKdv+nz6NLy8SGrnQJCC9RzvZF-QOdjxyZA () mail ! gmail ! com
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Hi Florian,

On Mon, Jul 22, 2019 at 9:28 PM Ben Cotton <bcotton@redhat.com> wrote:
>
> https://fedoraproject.org/wiki/Changes/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
> <code>x86_64</code> 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 ==
> * Name: [[User:fweimer| Florian Weimer]]
> * Email: [mailto:fweimer@redhat.com fweimer@redhat.com]
>
> == 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 [https://en.wikipedia.org/wiki/Advanced_Vector_Extensions#CPUs_with_AVX2
> 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.

It seems that Intel is still manufacturing CPUs without AVX support
(not even talking about AVX2) in 2019. So this is clearly no-go for
me.

But I do want to see some refreshments in this area! There are
multiple options how to proceed I think:

1. Lower requirement to something like SSE4 and select other CPU
features which are available in most of CPUs for last decade.
2. Build every package on x86_64 twice (one for compatible set and one
for this new-features set), possibly by introducting sub-architecture
in koji or using koji-shadow (that's just implementation detail.
Produce an official spin which is built from these packages.
3. Invent some mechanism for selecting appropriate feature set in
runtime (somebody mentioned fat binaries in this thread).

These options can be combined.

>
> 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
> <code>-latomic</code>, 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 <code>gcc</code> and
> <code>redhat-rpm-config</code> package to implement the new compiler
> flags.  It is expected that the new baseline will be implemented in a
> new GCC <code>-march=</code> option for convenience.
>
> * Other developers: Other developers may have to adjust test suites
> which expect exact floating point results, and correct linking with
> <code>libatomic</code>. They will also have to upgrade their x86-64
> machines to something that can execute AVX2 instructions.
>
> * Release engineering: [https://pagure.io/releng/issue/8513 #8513]
> ** All Fedora builders need to be AVX2-capable.
> ** Infrastructure ticket:
> [https://pagure.io/fedora-infrastructure/issue/7968 #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 glags.
> * 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
> <code>grep avx2 /proc/cpuinfo</code>.
>
> --
> Ben Cotton
> He / Him / His
> Fedora Program Manager
> Red Hat
> TZ=America/Indiana/Indianapolis
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