Very long allocating memory or initializating by random_symmetric about fluidx3d HOT 3 CLOSED

Hi @rodionstepanov,

I can replicate this issue, full 8 minutes for 1024x512x512 on 2x Xeon E5-2680 v3, 48 threads. Digging in the internet, I found the root cause: gcc's std::rand() is dog slow with multithreading, as it contains a mutex lock.

If I change the initialization loop in main_setup() from

parallel_for(lbm.get_N(), [&](ulong n) { ... });

to

for(ulong n=0ull; n<lbm.get_N(); n++) { ... }

to run single-threaded, it's down to 2 minutes from 8.

I have now replaced std::rand() with the standard C99 LCG function; this is suitable for multithreading and brings multithreaded initialization down to 1 minute. Please update!

Kind regards,
Moritz

from fluidx3d.

@ProjectPhysX , thanks a lot. I've updated and it is fast and furious now 🥇

from fluidx3d.

Hi @rodionstepanov,

interesting observation, I wasn't aware of this. random_symmetric() is no more than a std::rand() call internally. During initialization you should see the CPU go to full load on all cores, on both systems; I added this multithreading in update 2.9. Maybe the CPU in your Linux system is weaker? Or maybe the rand() call with g++ does something weird? I'll test this myself and try to replicate.

Thanks and kind regards,
Moritz

from fluidx3d.