[Beowulf] C++ compilers and assembly

Benson Muite benson.muite at ut.ee
Mon Sep 10 01:18:55 PDT 2018

Thanks. This is interesting. Possibly also of interest:
Not for HPC, but some aspects look useful.

On 09/10/2018 06:50 AM, John Hearns via Beowulf wrote:
> Chris Samuels recent post reminds me.
> I went to a fascinating and well delivered talk by Jason Hearne McGuiness
> https://www.meetup.com/ACCULondon/events/253570550/
> https://accu.org/index.php/accu_branches/accu_london
> Slides are here:
> https://github.com/acculondon/2018-September
> I would encourage everyone to give this presentation at least a browse.
> Jason has done a lot of work in comparing the assembler code emitted
> by C++ compilers for simple statement blocks. Even within the g++
> series there are wildly different assembler sequences emitted. And of
> course differences between Intel and Clang.
> I would have naively assumed that within g+++ at least as improvements
> are made then there would be some commonality - ie an evolution. But I
> would be wrong.
> Jason work in HFT so the default path loaded in IF statements (for
> instance) is  important for low latency.
> Jason also did a lot of work in comparing performance before and after
> the Spectre/Meltdown fixes.  There is a huge amount of work gone into
> that.
> One other takeaway - the choice of Linux OS is critical. Really.
> Common wisdom is that OS choice accounts for approx. 4% of performance
> difference.
> Not so.
> There isa big difference between CentOS 6 and 7 (7 being better) and
> Ubuntu does not shine at all well on his FIX benchmarks. Gentoo does
> well - Jason being a fan of Gentoo.
> As I remember on these benchmarks OS choice can affect results by 20% !!!!
> Lastly Jason mentioned IncludeOS http://www.includeos.org/
> these gusy are implementing something I have tossed about on this list
> in the past - running applications in a lightweight OS without all the
> overhead of a multitasking system.
> I dont know that much about it - is anyone from that company on the list?
> John Hearns
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