We discuss the inter-process communication in software distributed shared memory (S-DSM) systems. Some S-DSM systems, such as TreadMarks and JIAJIA, adopt the user datagram protocol (UDP) which does not provide the reliable communication between the computation nodes. To detect a communication error and recover from it, therefore, an acknowledgment is used for every message transmission in the middleware layer. In this paper, first, we show that an acknowledgment is not necessarily required for each message transmission in the middleware layer. Second, a method to reduce the acknowledgment overhead for a page request is proposed. We implemented the proposed method in our S-DSM system Mocha. The performance of the method was measured with several benchmark programs on both a PC cluster and an SMP cluster.