DASH is a distributed operating system kernel. Message-passing (MP) is used for local communication, and the MP system uses virtual memory remapping instead of software copying for moving large amounts of data between virtual address spaces. This design eliminates a bottleneck in high-performance communication, and increases the feasibility of moving services such as file services into user spaces.Other systems that have used VM remapping for message transfer have suffered from high per-operation overhead, limiting the use of the technique. The DASH design is intended to reduce this overhead. To evaluate our design, we measured the performance of the DASH kernel implementation on Sun 3/50 workstations. Our throughput measurements show that large messages can be moved between user spaces at a rate of more than 30 MB/sec, an order of magnitude higher than with software copying. Furthermore, the per-operation overhead is low, so performance for small messages is not sacrificed.To further understand the performance of the DASH MP system, we then broke an MP operation into short code segments and timed them with microsecond precision. The results show the relative costs of data movement and the other components of MP operations, and allow us to evaluate several specific design decisions.
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