  We present ab initio global general-relativistic Particle-in-cell (GR-PIC)
simulations of compact millisecond neutron star magnetospheres in the
axisymmetric aligned rotator configuration. We investigate the role of GR and
plasma supply on the polar cap particle acceleration efficiency - the precursor
of coherent radio emission - employing a new module for the PIC code OSIRIS,
designed to model plasma dynamics around compact objects with fully
self-consistent GR effects. We provide a detailed description of the main
sub-algorithms of the novel PIC algorithm, including a charge-conserving
current deposit scheme for curvilinear coordinates. We demonstrate efficient
particle acceleration in the polar caps of compact neutron stars with denser
magnetospheres, numerically validating the spacelike current extension provided
by force-free models. We show that GR relaxes the minimum required poloidal
magnetospheric current for the transition of the polar cap to the accelerator
regime, thus justifying the observation of weak pulsars beyond the expected
death line. We denote that spin-down luminosity intermittency and radio pulse
nullings for older pulsars might arise from the interplay between the polar and
outer gaps. Also, narrower radio beams are expected for weaker low-obliquity
pulsars.
