Hi folks!

I'm adding a CC to Steve Capper, a colleague in Arm who's our expert
here for this kind of question. He's also a DM in Debian... :-)

On Tue, Apr 21, 2020 at 06:37:07PM -0400, Noah Meyerhans wrote:
>On Sun, Apr 12, 2020 at 12:18:35PM +0200, Aurelien Jarno wrote:
>> It would also be nice to have numbers to see the impact on non-ARMv8.1
>> CPU on real workloads. As pointed out by Florian, and if the impact is
>> negligible, it might be a good idea to enable -moutline-atomics
>> globally at the GCC level so that all software can benefit from it, and
>> instead of only glibc. That could be either upstream or only in Debian,
>> that's probably a separate discussion. Otherwise we will likely end up
>> using this non-default GCC option on all packages that runs faster with
>> it.

I think the -moutline-atomics is probably good to enable by default
once we've got it (gcc 10). that's the suggestion I've heard from gcc
folks in Arm.

>> Also note that the mechanism allowing a safe upgrade *does* incur a 
>> runtime overhead as every binary now has to test for the presence of
>> /etc/ld.so.nohwcap to detect a possible upgrade of the glibc in
>> progress. That's why we have disabled it on architecture not providing
>> an optimized library [1].

Oh, ick. :-/

>Thanks for the pointer, it's interesting to see data on that.  This also
>suggests that it might be worthwhile to investigate a better mechanism
>for identifying the availability of hardware features.
>> > I've tested both options and found them to be acceptable on v8.1a (Neoverse
>> > N1) and v8a (Cortex A72) CPUs.  I can provide bulk test run data of the
>> > various different configuration permutations if you'd like to see 
>> > additional
>> > data.
>> As said above I think we would need more numbers on real workload to
>> take a decision. Don't get me wrong I do not oppose on improving atomics
>> on ARMv8.1, but I would like that we chose the best option. Also if we
>> go with the -moutline-atomics option, I believe it rather has to be a
>> ARM porters decision than a glibc maintainers decision (hence the Cc:).
>I'll see what I can come up with.
>Do the arm porters have any opinions on this matter?

It's a good question, and thanks for asking! I definitely think it's
worth doing -moutline-atomics, and I'm hoping Steve can share some
performance numbers to help convince. :-)

Steve McIntyre, Cambridge, UK.                                st...@einval.com
Who needs computer imagery when you've got Brian Blessed?

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