[Interest] Slow graphics effect (due to _dl_resolve_runtime_avx)

[Thiago Macieira]

On sábado, 26 de novembro de 2016 13:01:03 PST Ch'Gans wrote:
> On 26 November 2016 at 11:57, Thiago Macieira <thiago.macieira at intel.com> 
wrote:
> > That's a useless result. Your tool is telling you not the proper time of
> > each function, but the aggregate time of that function and everything it
> > called.
> Well, I thought that maybe the role of _dl_runtime_resolv_avx was to
> actually only resolve the function, not execute it as well...

Yes, and after resolving that function, it calls that function.

> > It's like saying that your application spends most of its time in main. We
> > knew that... Drill down and find out what's happening. Very likely, the
> > effect you turned on is too slow for your CPU or GPU, or in combination
> > with some other system-dependent use.
> 
> /proc/cpu tells me that the CPU supports AVX (no trace of AVX2). So
> could it be a bug somewhere. basically the wrong implementation is
> called, or my CPU is not good enough, well it's a modern AMD:  "AMD
> FX(tm)-8320 Eight-Core Processor".

That explains why _dl_runtime_resolve_avx was used, as opposed to the non-AVX 
version of it. 

Note that I can't find that function in glibc's source code. If it really is a 
problem, then your distribution caused it by applying a patch and you should 
complain to them. But I don't think that this the problem. Stop looking at 
_dl_runtime_resolve_avx, that's a red herring. Your problem is elsewhere.

> What I'm trying to find out, is that is it a Qt problem (relying on
> AVX2 on a CPU that doesn't support it), or is it a KUbuntu problem, or
> ...

I'll take the "..." as the problem.

> As well, is there a simple way to disable AVX2 within Qt?

Yes. But it's already disabled. And that's not the problem.

> I find it a bit crazy that Qt graphics performance are that bad on a
> modern desktop computer using latest (or almost) AMD CPU.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center