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List:       fedora-devel-list
Subject:    Re: Fedora 32 System-Wide Change proposal: x86-64 micro-architecture update
From:       Dave Love <loveshack () fedoraproject ! org>
Date:       2019-07-31 20:58:01
Message-ID: 87wofxudk6.fsf () albion ! it ! manchester ! ac ! uk
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Frantisek Zatloukal <fzatlouk@redhat.com> writes:

> On Wed, Jul 31, 2019 at 11:00 AM Kevin Kofler <kevin.kofler@chello.at>
> wrote:
>
>> * the performance increase to be had is marginal, given that we are mostly
>>   talking about code written in C or C++ without even compiler
>> vectorization
>>   (-ftree-vectorize) turned on,
>>
>
> Are you sure? Fore example (and there are more of them), lots of these do
> not seem marginal:
> https://www.phoronix.com/scan.php?page=news_item&px=Clear-Linux-2019-Python-Perf
>  , https://www.phoronix.com/scan.php?page=article&item=linux-2016-2018&num=3

I see typically useless benchmarks without enough information, or
profiles, that provide no real insight.  These things rarely measure
what they purport to; also error bars -- we've heard of them.  Numpy is
presumably dominated by level 3 BLAS (a library which is swappable on
Ubuntu, as it should be in Fedora, with potentially two orders of
magnitude performance difference in DGEMM), and whatever threading is
used.  I suspect similarly for the other things.  First take Intel
proprietary stuff out of the equation, and think about those numbers
taken at face value.

Note that using avx2 can be worse than sse2/4, and cache effects are
often more important (as in optimized BLAS).
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