  In the upcoming upgrades for Run 3 and 4, the LHC will significantly increase
Pb--Pb and pp interaction rates. This goes along with upgrades of all
experiments, ALICE, ATLAS, CMS, and LHCb, related to both the detectors and the
computing. The online processing farms must employ faster, more efficient
reconstruction algorithms to cope with the increased data rates, and data
compression factors must increase to fit the data in the affordable capacity
for permanent storage. Due to different operating conditions and aims, the
experiments follow different approaches, but there are several common trends
like more extensive online computing and the adoption of hardware accelerators.
This paper gives an overview and compares the data processing approaches and
the online computing farms of the LHC experiments today in Run 2 and for the
upcoming LHC Run 3 and 4.
