Several manufacturers have recently announced the first
simultaneous-multithreaded processors, both as single CPUs and as
components of multi-CPU chips. All are small scale, comprising only
two to four thread contexts. A significant impediment to the construction of
larger-scale SMTs is the register file size required by a large number of
contexts. This paper introduces and evaluates mini-threads, a simple
extension to SMT that increases thread-level parallelism without the
commensurate increase in register file size. A mini-threaded SMT CPU adds
additional per-thread state to each hardware context; an application
executing in a context can create mini-threads that will utilize its own
per-thread state, but share the context=92s architectural register set. The
resulting performance will depend on the benefits of additional TLP
compared to the costs of executing mini-threads with fewer registers. Our
results quantify these factors in detail and demonstrate that mini-threads
can improve performance significantly, particularly on small-scale,
space-sensitive CPU designs.