Used perf record for getting CPU efficiency numbers, and it worked very well, however to be able to reliably trace the code, I had to compile it in debug mode with no optimizations, and run with a single thread. That combination increased the execution time considerably.

Some of the Matrix and solver code coming from Eigen is heavily optimized for SIMD operations and using it with -O0 is just painful.

Moreover, I had to verify its memory sanity and used Valgrind for that. Using a full size problem meant it had to churn for days to finish execution.

Having deadlines doesn't always help in this stuff.