The current generation of time-based transactional memo-
ries (TMs) has the advantage of being simple and efficient,
and providing strong linearizability semantics. Linearizabil-
ity matches well the goal of TM to simplify the design and
implementation of concurrent applications. However, long
transactions can have a much lower likelihood of committing
than smaller transactions because of the strict ordering con-
straints imposed by linearizability. We investigate the use of
weaker semantics for TM and introduce a new consistency
criterion that we call z-linearizability. By combining proper-
ties of linearizability and serializability, z-linearizability pro-
vides a good trade-off between strong semantics and good
practical performance even for long transactions.