c++multithreadingopencvopenmp

Is opencv::flann:Index knnsearch thread safe?

I am trying to parallelize my ML problem with flann Index. My code in nutshell looks like this:

Index index(dict, KDTreeIndexParams(TREE_NUM)); // HUGE dict, very expensive to construct -- prefer to create it once.
#pragma omp parallel for
for (int i = 0; i < featuresize; i++) {
    // creating thread-local params
    auto denseTF = index.knnSearch(<thread-local params>);
    // not relevant code
}

I looked up documentation here but there is nothing about thread safety there. My concern is whether this snippet is thread safe?

Solution

  • After a day of debugging and catching a data race and reading through source code (here ) I can conclude that index.knnSearch is NOT thread safe.

    Internally, indexTree is being updated on knnSearch call. I solved this issue by creating a copy of index for each thread (yes, it is expensive, but still faster than sequential code).