To program massively concurrent MIMD machines, programmers need tools for managing complexity. One important tool that has been used in the sequential programming world is hierarchies of abstractions. Unfortunately, most concurrent object-oriented languages construct hierarchical abstractions from objects that serialize --- serializing the abstractions. In machines with tens of thousands of processors, unnecessary serialization of this sort can cause significant loss of concurrency.Concurrent Aggregates (CA) is an object-oriented language that allows programmers to build unserialized hierarchies of abstractions by using aggregates. An aggregate in CA is a homogeneous collection of objects (called representatives) that are grouped together and may be referenced by a single aggregate name. Aggregates are integrated into the object model, allowing them to be used wherever an object could be used. Concurrent Aggregates also incorporates several innovative language features that facilitate programmingwith aggregates. Intra-aggregate addressing aids cooperation between parts of an aggregate. Delegation allows programmers to compose a concurrent aggregate behavior from a number of objects or aggregates. Messages in CA are first class objects that can be used to create message handling abstractions (they handle messages as data). Such abstractions facilitate concurrent operations on aggregates. Continuations are also first class objects. In addition, programmers can construct continuations and use them just like system continuations. User constructed continuations can implement synchronization structures such as a barrier synchronization.We have implemented Concurrent Aggregates and have been using the language since August 1989. We have written a number of significant application programs including a PC board router, a concurrent B-tree, a digital logic simulator, a parallel FIFO queue and a multi-grid solver. More information on Concurrent Aggregates can be found in [10, 11, 9].
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