  The factor of four increase in the LHC luminosity, from $0.5\times
10^{34}\,\textrm{cm}^{-2}\textrm{s}^{-1}$ to $2.0\times
10^{34}\textrm{cm}^{-2}\textrm{s}^{-1}$, and the corresponding increase in
pile-up collisions during the 2015-2018 data-taking period, presented a
challenge for ATLAS to trigger on missing transverse momentum. The output data
rate at fixed threshold typically increases exponentially with the number of
pile-up collisions, so the legacy algorithms from previous LHC data-taking
periods had to be tuned and new approaches developed to maintain the high
trigger efficiency achieved in earlier operations. A study of the trigger
performance and comparisons with simulations show that these changes resulted
in event selection efficiencies of >98% for this period, meeting and in some
cases exceeding the performance of similar triggers in earlier run periods,
while at the same time keeping the necessary bandwidth within acceptable
limits.
