  In this study, the gravitational octree code originally optimized for the
Fermi, Kepler, and Maxwell GPU architectures is adapted to the Volta
architecture. The Volta architecture introduces independent thread scheduling
requiring either the insertion of the explicit synchronizations at appropriate
locations or the enforcement of the same implicit synchronizations as do the
Pascal or earlier architectures by specifying \texttt{-gencode
arch=compute\_60,code=sm\_70}. The performance measurements on Tesla V100, the
current flagship GPU by NVIDIA, revealed that the $N$-body simulations of the
Andromeda galaxy model with $2^{23} = 8388608$ particles took $3.8 \times
10^{-2}$~s or $3.3 \times 10^{-2}$~s per step for each case. Tesla V100
achieves a 1.4 to 2.2-fold acceleration in comparison with Tesla P100, the
flagship GPU in the previous generation. The observed speed-up of 2.2 is
greater than 1.5, which is the ratio of the theoretical peak performance of the
two GPUs. The independence of the units for integer operations from those for
floating-point number operations enables the overlapped execution of integer
and floating-point number operations. It hides the execution time of the
integer operations leading to the speed-up rate above the theoretical peak
performance ratio. Tesla V100 can execute $N$-body simulation with up to $25
\times 2^{20} = 26214400$ particles, and it took $2.0 \times 10^{-1}$~s per
step. It corresponds to $3.5$~TFlop/s, which is 22\% of the single-precision
theoretical peak performance.
