  In this proceeding, we present our recent investigations on hydrodynamic
collectivity in high-multiplicity proton--proton collisions at $\sqrt{s}=$ 13
TeV using the VISHNU hybrid model with different initial condition models,
called HIJING, super-MC and TRENTo. We find that with carefully tuned
parameters, hydrodynamic simulations can give reasonable descriptions of the
measured two-particle correlations. However, multi-particle single and mixed
harmonics cumulants can not be described by hydrodynamics with these three
initial conditions, even for the signs in a few cases. Further studies show
that the non-linear response plays an important role in the hydrodynamic
expansion of the p--p systems. Such an effect can change $c_2\{4\}$ from a
negative value in the initial state to a positive value in the final state. The
failure of the hydrodynamic description of multi-particle cumulant triggers the
questions on whether the hydrodynamics can rule all collision systems,
including p--p collisions at the LHC.
