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Date: Sun, 16 Aug 2015 22:41:37 +0300
From: Solar Designer <>
Subject: Re: FMT_OMP_BAD

On Sun, Aug 16, 2015 at 08:05:42PM +0300, Solar Designer wrote:
> We may agree on two cut-off points: e.g. < 4.0 (out of ideal 10.0) for
> disabling OpenMP in default builds, and < 7.0 for setting FMT_OMP_BAD.

Actually, ideal at this test on this machine is less than 10.0, because
the CPU clock rate changes.  For 1 active core, it is 3.3 GHz.  For 10
active cores (hopefully, 5 out of 8 per chip), it's most likely 3.1 or
3.0 GHz.  So ideal is more like:

3.1*10/3.3 = 9.39

and this means that quite a few formats demonstrated unexpectedly good
scaling.  This indicates that they're (more) suboptimal when running 1

E5-2670 has turbo steps 4/4/5/5/6/6/7/7, where each corresponds to extra
100 MHz on top of the base frequency of 2.6 GHz.  So it's most likely
+700 MHz vs. +500 MHz here.


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