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. > >Agreed. 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 . 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?