Книга: Distributed operating systems

6.4. PAGE-BASED DISTRIBUTED SHARED MEMORY

Having studied the principles behind distributed shared memory systems, let us now turn to these systems themselves. In this section we will study "classical" distributed shared memory, the first of which was IVY (Li 1986; and Li and Hudak 1989). These systems are built on top of multicomputers, that is, processors connected by a specialized message-passing network, workstations on a LAN, or similar designs. The essential element here is that no processor can directly access any other processor's memory. Such systems are sometimes called NORMA (NO Remote Memory Access) systems to contrast them with NUMA systems.

The big difference between NUMA and NORMA is that in the former, every processor can directly reference every word in the global address space just by reading or writing it. Pages can be randomly distributed among memories without affecting the results that programs give. When a processor references a remote page, the system has the option of fetching it or using it remotely. The decision affects the performance, but not the correctness. NUMA machines are true multiprocessors — the hardware allows every processor to reference every word in the address space without software intervention.

Workstations on a LAN are fundamentally different from a multiprocessor. Processors can only reference their own local memory. There is no concept of a global shared memory, as there is with a NUMA or UMA multiprocessor. The goal of the DSM work, however, is to add software to the system to allow a multicomputer to run multiprocessor programs. Consequently, when a processor references a remote page, that page must be fetched. There is no choice as there is in the NUMA case.

Much of the early research on DSM systems was devoted to the question of how to run existing multiprocessor programs on multicomputers. Sometimes this is referred to as the "dusty deck" problem. The idea is to breathe new life into old programs just by running them on new (DSM) systems. The concept is especially attractive for applications that need all the CPU cycles they can get and whose authors are thus interested in using large-scale multicomputers rather than small-scale multiprocessors.

Since programs written for multiprocessors normally assume that memory is sequentially consistent, the initial work on DSM was carefully done to provide sequentially consistent memory, so that old multiprocessor programs could work without modification. Subsequent experience has shown that major performance gains can be had by relaxing the memory model, at the cost of reprogramming existing applications and writing new ones in a different style. We will come back to this point later, but first we will look at the major design issues in classical DSM systems of the IVY type.

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