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Paper Details

%T Higher Levels of Process Synchronisation
%A Peter H. Welch, David C. Wood
%E André W. P. Bakkers
%B Proceedings of WoTUG\-20: Parallel Programming and Java
%X Four new synchronisation primitives (SEMAPHOREs, RESOURCEs,
   EVENTs and BUCKETs) were introduced in the KRoC 0.8beta
   release of occam for SPARC (SunOS/Solaris) and Alpha (OSF/1)
   UNIX workstations. This paper reports on the rationale,
   application and implementation of two of these (SEMAPHOREs
   and EVENTs). Details on the other two may be found on the
   web. The new primitives are designed to support
   higher\-level mechanisms of SHARING between parallel
   processes and give us greater powers of expression. They
   will also let greater levels of concurrency be safely
   exploited from future parallel architectures, such as those
   providing (virtual) shared\-memory. They demonstrate that
   occam is neutral in any debate between the merits of
   message\-passing versus shared\-memory parallelism, enabling
   applications to take advantage of whichever paradigm (or
   mixture of paradigms) is the most appropriate. The new
   primitives could be (but are not) implemented in terms of
   traditional channels, but only at the expense of increased
   complexity and computational overhead. The primitives are
   immediately useful even for uni\-processors \-\- for
   example, the cost of a fair ALT can be reduced from O(n) to
   O(1). In fact, all the operations associated with new
   primitives have constant space and time complexities; and
   the constants are very low. The KRoC release provides an
   Abstract Data Type interface to the primitives. However,
   direct use of such mechanisms still allows the user to
   misuse them. They must be used in the ways prescribed below
   else else their semantics become unpredictable. No tool is
   provided to check correct usage at this level. The intention
   is to bind those primitives found to be useful into higher
   level versions of occam. Some of the primitives (e.g.
   SEMAPHOREs) may never themselves be made visible in the
   language, but may be used to implement bindings of
   higher\-level paradigms (such as SHARED channels and
   BLACKBOARDs). The compiler will perform the relevant usage
   checking on all new language bindings, closing the security
   loopholes opened by raw use of the primitives. The paper
   closes by relating this work with the notions of virtual
   transputers, microcoded schedulers, object orientation and
   Java threads.

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