  A distributed multi-writer multi-reader (MWMR) atomic register is an
important primitive that enables a wide range of distributed algorithms. Hence,
improving its performance can have large-scale consequences. Since the seminal
work of ABD emulation in the message-passing networks [JACM '95], many
researchers study fast implementations of atomic registers under various
conditions. "Fast" means that a read or a write can be completed with 1
round-trip time (RTT), by contacting a simple majority. In this work, we
explore an atomic register with optimal resilience and "optimistically fast"
read and write operations. That is, both operations can be fast if there is no
concurrent write.
  This paper has three contributions: (i) We present Gus, the emulation of an
MWMR atomic register with optimal resilience and optimistically fast reads and
writes when there are up to 5 nodes; (ii) We show that when there are > 5
nodes, it is impossible to emulate an MWMR atomic register with both
properties; and (iii) We implement Gus in the framework of EPaxos and Gryff,
and show that Gus provides lower tail latency than state-of-the-art systems
such as EPaxos, Gryff, Giza, and Tempo under various workloads in the context
of geo-replicated object storage systems.
