Software DSM systems provide good programmability for cluster computing. Yet it does not gain as wide acceptance as it should, because of performance issue and the size limitation of shared memory space available for large applications. This paper introduces LOTS, a C++ runtime library supporting a large shared object space through the dynamic memory mapping mechanism. LOTS allows more objects to be mapped in the process space, since only a trace of control information for each object is resident locally, while the object data is mapped lazily from the local disk to the virtual memory during access. To our knowledge, LOTS is the first pure runtime software DSM supporting a shared object space larger than the local process space. Our testing shows that LOTS can allocate 117.77 GB of shared objects on a cluster of 4 machines, utilizing all the free hard disk space available. LOTS only pays a small overhead on supporting large object space, but with its scope consistency and a new mixed coherence protocol, LOTS achieves better scalability on problem size and cluster size. It also retains good programmability as most of the C++ programming syntax is preserved. Content-Disposition: form-data; name="OLDPASSWORD"