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Date: Sun, 16 Aug 2015 22:41:37 +0300
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
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
thread.
https://en.wikipedia.org/wiki/List_of_Intel_Xeon_microprocessors#Xeon_E5-26xx_.28dual-processor.29
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.
Alexander
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