  The ALICE HLT uses a data transport framework based on the
publisher-subscriber message principle, which transparently handles the
communication between processing components over the network and between
processing components on the same node via shared memory with a zero copy
approach. We present an analysis of the performance in terms of maximum
achievable data rates and event rates as well as processing capabilities during
Run 1 and Run 2. Based on this analysis, we present new optimizations we have
developed for ALICE in Run 2. These include support for asynchronous transport
via Zero-MQ which enables loops in the reconstruction chain graph and which is
used to ship QA histograms to DQM. We have added asynchronous processing
capabilities in order to support long-running tasks besides the
event-synchronous reconstruction tasks in normal HLT operation. These
asynchronous components run in an isolated process such that the HLT as a whole
is resilient even to fatal errors in these asynchronous components. In this
way, we can ensure that new developments cannot break data taking. On top of
that, we have tuned the processing chain to cope with the higher event and data
rates expected from the new TPC readout electronics (RCU2) and we have improved
the configuration procedure and the startup time in order to increase the time
where ALICE can take physics data. We analyze the maximum achievable data
processing rates taking into account processing capabilities of CPUs and GPUs,
buffer sizes, network bandwidth, the incoming links from the detectors, and the
outgoing links to data acquisition.
