Parallel Algorithms with the Desired Sequential Workload

Could you share a known algorithm that relies on the following models?

# 1

# 2

These models have a multithreaded and sequential workload. These algorithms must be joined to perform sequential work. Then they can enter the new multithreaded region (# 2) where the program can exit (# 1). The multiplication of matrix chains can be an example. The parentheses are placed sequentially, and the multiplication is performed in a multithreaded region.

Basically, at least 10% of the total run time must be sequential in these algorithms. Synthetic algorithms are not welcome :), known algorithms must be used. Can any one suggest an algorithm?