Книга: Distributed operating systems

9.7.4. Memory Model

9.7.4. Memory Model

Amoeba's memory model is based on variable-length segments. A virtual address space consists of some number of segments mapped in at specific addresses. Segments can be mapped in and out at will. Each segment is controlled by a capability. A remote process that has the capabilities for another process' segments (e.g., a debugger) can read and write them from any other Amoeba machine. Amoeba does not support paging. When a process is running, all of its segments are in memory. The ideas behind this decision are simplicity of design and high performance, coupled with the fact that extremely large memories are becoming common on even the smallest machines.

Mach's memory model is based on memory objects and is implemented in terms of fixed-size pages. Memory objects can be mapped and unmapped at will. A memory object need not be entirely in main memory to be used. When an absent page is referenced, a page fault occurs and a message is sent to an external memory manager to find the page and map it in. Together with the default memory manager, this mechanism supports paged virtual memory.

Pages can be shared between multiple processes in various ways. One common configuration is the copy-on-write sharing used to attach a child process to its parent. Although this mechanism is a highly efficient way of sharing on a single node, it loses its advantages in a distributed system because physical transport is always required (assuming that the receiver needs to read the data). In such an environment, the extra code and complexity are wasted. This is a clear example of where Mach has been optimized for single-CPU and multiprocessor systems, rather than for distributed systems.

Chorus' memory management model is taken largely from Mach. It too has memory objects (segments) that can be mapped in. These are demand paged under the control of an external pager (mapper), as in Mach.

Amoeba, Mach, and Chorus all support distributed shared memory, but they do it in different ways. Amoeba supports shared objects that are replicated on all machines using them. Objects can be of any size and can support any operations. Reads are done locally, and writes are done using the Amoeba reliable broadcast protocol.

Mach and Chorus, in contrast, support a page-based distributed shared memory. When a thread references a page that is not present on its machine, the page is fetched from its current machine and brought in. If two machines heavily access the same writable page, thrashing can occur. The trade-off here is the more expensive update on Amoeba (due to the replication of writable objects), versus the potential for thrashing on Mach and Chorus (only one copy of writable pages).

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