Using openmp timing omp_get_wtime() breaks ITensor 2 calculations

Question

Hello,

I stumbled upon a problem in conjecture with openMP and ITensor2. It may very well have nothing to do with itensor, so I apologize in advance. Googling brought me nowhere, so I hope someone here may be able to figure out what the problem is.

Basically, adding a call to omp_get_wtime() and running the program with more than a single thread completely messes up some itensor calculations.

I copied a minimal example at the end of this message. I am using the latest itensor 2 (did git pull) and LAPACK, c++11, compiling using -fopenmp.
I was only able to reproduce the problem with more than a single thread (for example, export OMP_NUM_THREADS=2). When setting export OMP_NUM_THREADS=1 , things seem to work normally.
Similarly, If I remove the double start = omp_get_wtime(); line, things seem to work properly.

Here is a minimal example:

#include "itensor/all.h"
#include <omp.h>


using namespace itensor;

int 
main()
    {
    int N = 10;

    auto sites = SpinOne(N);

    auto ampo = AutoMPO(sites);
    for(int j = 1; j < N; ++j)
        {
        ampo += 0.5,"S+",j,"S-",j+1;
        ampo += 0.5,"S-",j,"S+",j+1;
        ampo +=     "Sz",j,"Sz",j+1;
        }
    auto H = MPO(ampo);

    auto sweeps = Sweeps(5); //number of sweeps is 5
    sweeps.maxm() = 10,20,100,100,200;
    sweeps.cutoff() = 1E-10;

    auto psi = MPS(sites);
    double start = omp_get_wtime(); 

    auto energy = dmrg(psi,H,sweeps,{"Quiet=",true});

    printfln("Ground state energy from DMRG = %.12f",energy);


    return 0;
    }

Thanks

Answer 1

That code is running fine for me in ITensor v2 and v3 for both 1 and 2 threads. I am using ITensor v2 and v3 compiled with MKL. What errors are you seeing?