  A new generation of laser wakefield accelerators, supported by the extreme
accelerating fields generated in the interaction of PW-Class lasers and
underdense targets, promises the production of high quality electron beams in
short distances for multiple applications. Achieving this goal will rely
heavily on numerical modeling for further understanding of the underlying
physics and identification of optimal regimes, but large scale modeling of
these scenarios is computationally heavy and requires efficient use of
state-of-the-art Petascale supercomputing systems. We discuss the main
difficulties involved in running these simulations and the new developments
implemented in the OSIRIS framework to address these issues, ranging from
multi-dimensional dynamic load balancing and hybrid distributed / shared memory
parallelism to the vectorization of the PIC algorithm. We present the results
of the OASCR Joule Metric program on the issue of large scale modeling of LWFA,
demonstrating speedups of over 1 order of magnitude on the same hardware.
Finally, scalability to over $\sim 10^6$ cores, and sustained performance over
$\sim 2$ PFlops is demonstrated, opening the way for large scale modeling of
laser wakefield accelerator scenarios.
