  We present a fast solver for the 3D high-frequency Helmholtz equation in
heterogeneous, constant density, acoustic media. The solver is based on the
method of polarized traces, coupled with distributed linear algebra libraries
and pipelining to obtain an empirical online runtime $ \mathcal{O}(\max(1,R/n)
N \log N)$ where $N = n^3$ is the total number of degrees of freedom and $R$ is
the number of right-hand sides. Such a favorable scaling is a prerequisite for
large-scale implementations of full waveform inversion (FWI) in frequency
domain.
