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CS COLLOQUIUM

Predictable Threads for Dynamic, Hard
Real-Time Environments

Marty Humphrey
Department of Computer Science
and Engineering
University of Colorado
Denver, Colorado

November 7, 1996
3:45 - 4:45 pm
Engineering Center Classroom 265
Refreshments will be served at 3:30 pm

The key metric of a real-time operating system is its predictability:
kernel operations must not take an arbitrary amount of time, and there must
exist support for the predictable execution of application code.  Most
existing real-time thread packages are based on modifications to
non-real-time kernels that support threads.  In these systems, meeting hard
real-time deadlines can be difficult---because the thread interacts with
the non-real-time components of the kernel, performance can vary from
instance to instance.

The UMass Spring threads package has been designed to achieve
predictability in hard real-time environments.  The first part of the talk
focuses on the threads package specification (similar in principle to POSIX
PThreads and Real-Time Extensions).  The contribution of this work is
support for new semantics ("guaranteed" and "nonguaranteed" thread group
spawning) and explicit support for semantically-correct premature thread
(and thread group) termination under hard real-time constraints.

The second part of the talk discusses the implementation of the design
specification in the UMass Spring kernel.  Spring is a parallel and
distributed operating system featuring an on-line, planning-based scheduler
that makes it suitable for dynamic environments.  Prior to his work, Spring
applications could utilize only a single flow of execution (similar to a
UNIX-like process model, adapted for real-time).  A system description
language and real-time programming language are used to meet the
requirements of the threads package specification.  Challenges and design
decisions of the implementation of the threads package for a multiprocessor
environment will be discussed.  The evaluation of the implementation of the
threads package in the Umass Spring kernel is discussed both in terms of
primitive operations and higher-level system activities.  In addition to
new flexibility for user and kernel applications, the adoption of the
threads package in the UMass Spring kernel significantly reduces kernel
overhead in the context switch and memory access time.  Results show that
the lack of variance makes this package, to our knowledge, the first
threads package suitable for dynamic, hard real-time environments.

Host:  Dirk Grunwald
------------------------------
http://www.cs.colorado.edu/~grunwald/Colloquia.html



***********************************
Pat Libhart
Department of Computer Science
University of Colorado,  ECOT 717
Campus Box 430, Boulder, CO  80309-0430
Phone:  (303)492-7514     Fax:  (303)492-2844
e-mail:  libhart@cs.colorado.edu
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