  In this talk, we report on our results about the computation of the elliptic
flow of the quark-gluon-plasma produced in relativistic heavy ion collisions,
simulating the expansion of the fireball by solving the relativistic Boltzmann
equation for the parton distribution function tuned at a fixed shear viscosity
to entropy density ratio $\eta/s$. We emphasize the role of saturation in the
initial gluon spectrum modelling the shattering of the color glass condensate,
causing the initial distribution to be out of equilibrium. We find that the
saturation reduces the efficiency in building-up the elliptic flow, even if the
thermalization process is quite fast $\tau_{therm} \approx 0.8 \,\rm fm/c$. and
the pressure isotropization even faster $\tau_{isotr} \approx 0.3 \,\rm fm/c$.
The impact of the initial non-equilibrium manifests for non-central collisions
and can modify the estimate of the viscosity respect to the assumption of full
thermalization in $p_T$-space.
