Doug.Jensen@K.GP.CS.CMU.EDU (08/17/89)
E. Douglas Jensen (edj@cs.cmu.edu. edj@westford.ccur.com) Alpha
E. Douglas Jensen
Concurrent Computer Corp.
One Technology Way
Westford, MA 01886
508-392-2999
The Alpha Real-Time Decentralized Operating System
Alpha is an operating system for the mission-critical integration and
operation of large, complex, distributed, real-time systems. Only recently
have such systems become more common in industrial factory and plant
automation (e.g., automobile manufacturing), aerospace (e.g.,space
stations), and military (e.g., C3I) contexts. They differ substantially
from the more widely known timesharing systems, numerically oriented
supercomputers, and networks of personal workstations. More surprisingly,
they also depart significantly from traditional real-time systems, which are
for low-level sampled data monitoring and control. The most challenging
technical requirements dictated by this application domain are in the areas
of: satisfying real-time constraints despite the system's inherently
stochastic and nondeterministic nature; distributed programming and
system-wide (inter-node) resource management; robustness in the face of
failures and even attacks; and adaptability to a wide range of
ever-changing requirements over decades of use. Satisfying these entails
unconventional design and implementation tradeoffs.
In Alpha's distributed programming model, activities correspond to threads,
which execute concurrently in otherwise passive objects, and cross object
(and, transparently and reliably, node) boundaries by means of operation
invocation; they carry with them attributes such as urgency, importance,
and relibility specified by the application. Alpha instances cooperate to
manage the global resources of the entire system based on these attributes,
using best-effort resource management algorithms to ensure that as many as
possible of the most important aperiodic as well as periodic time
constraints are met, permitting graceful degradation in response to the
inevitable overloads.
To facilitate maintaining integrity of system and application distributed
data and programs despite physical dispersal, asynchronous concurrency of
execution, and hardware failures, Alpha includes exception handling
facilities, thread repair, and kernel-level mechanisms for real-time atomic
transactions and object replication. Alpha uses policy/mechanism separation
to exploit application specificity in support of adaptability. Departing
from common practice, Alpha's performance is optimized for the important
high-stress exception cases, such as failure or attack, rather than for the
normal, most frequent cases.
Alpha embodies results from nine years of research performed by the Archons
Project at Carnegie Mellon University, where a prototype was built from 1984
to 1987; another copy has been successfully demonstrated with application
software written at General Dynamics Corp. Alpha research is ongoing at CMU
and other academic and industrial institutions, but is now led by Concurrent
Computer Corp., where it continues to be sponsored in part by DoD. A series
of next-generation designs and implementations will be delivered to various
Government and industry labs for experimental applications beginning in
early 1990.