Middleware Support for Aperiodic Tasks in Distributed Real-Time Systems
∗
Yuanfang Zhang, Chenyang Lu, and Christopher Gill
Department of Computer Science and Engineering
Washington University, St. Louis, MO, USA
{yfzhang, lu, cdgill}@cse.wustl.edu
Patrick Lardieri and Gautam Thaker
Advanced Technology Laboratories
Lockheed Martin, Cherry Hill, NJ, USA
{plardier, gthaker}@atl.lmco.com
Abstract
Many mission-critical distributed real-time applications
must handle aperiodic tasks with end-to-end deadlines.
However, existing middleware (e.g., RT-CORBA) lacks
schedulability analysis and run-time enforcement mecha-
nisms needed to give online real-time guarantees for ape-
riodic tasks. The primary contribution of this work is the
design, implementation, and performance evaluation of the
first realization of deferrable server and admission control
mechanisms for aperiodic tasks in middleware. Empirical
results on a KURT-Linux testbed demonstrate the efficiency
and effectiveness of our deferrable server and admission
control mechanisms in TAO’s federated event service.
1 Introduction
Many distributed real-time systems must handle a mix
of periodic and aperiodic tasks. Some aperiodic tasks have
end-to-end deadlines whose assurance is critical to the cor-
rect behavior of the system. For example, in an industrial
plant monitoring system, an aperiodic alert event may be
generated when a series of periodic sensor readings meets
certain hazard detection criteria. This event must be pro-
cessed on multiple processors within an end-to-end dead-
line. User inputs and sensor readings may trigger various
other real-time aperiodic tasks. A key challenge in such
systems is providing online real-time guarantees to critical
aperiodic tasks that arrive dynamically.
State of the Art. Schedulability analysis is essential
for achieving predictable real-time properties. Aperi-
odic scheduling has been studied extensively in real-time
scheduling theory. Earlier work on aperiodic servers has
integrated scheduling of aperiodic and periodic tasks [24,
19, 11, 21, 16, 17, 12, 5, 22], and new schedulability
∗
This work was supported in part by the DARPA Adaptive and Reflec-
tive Middleware Systems (ARMS) program (contract NBCHC030140) and
NSF CAREER award CNS-0448554.
tests based on aperiodic utilization bounds [2] and a new
admission control approach [3] were introduced recently.
However, despite significant theoretical results on aperi-
odic scheduling, these results have not been applied to the
standards-based middleware that is increasingly being used
for developing distributed real-time applications. For ex-
ample, current implementations of Real-Time CORBA (RT-
CORBA) [14] do not provide any of the schedulability tests
or run-time mechanisms required by aperiodic servers. As a
result, those middleware implementations are currently un-
suitable for applications with real-time aperiodic tasks. Ad-
mission control has been proposed as an effective approach
to handle dynamic real-time tasks in distributed operating
systems [23, 20]. However, those kernel-level mechanisms
cannot be ported to distributed middleware that lacks fine-
grained resource control. Admission control also has been
implemented in real-time middleware services [1], but they
do not support aperiodic end-to-end tasks, which are essen-
tial to many distributed real-time applications.
Research Contributions. To address the limitations of
current generation real-time middleware in supporting ape-
riodic tasks in dynamic distributed real-time systems, we
have developed an integrated middleware architecture for
end-to-end aperiodic and periodic task scheduling in The
ACE ORB (TAO) [9]. We have developed what are to our
knowledge the first middleware-layer (1) mechanisms for
deferrable servers, in TAO’s federated event service [8];
and (2) admission control service supporting both aperiodic
and periodic end-to-end tasks. Our work bridges an im-
portant gap between aperiodic scheduling theory and state-
of-the-art real-time middleware. Applying that theory to a
real-world middleware environment faces several important
challenges as it requires (1) consideration of the specific re-
quirements of the system model in applying that theory, and
(2) highly efficient design and implementation of the run-
time mechanisms on standard operating system platforms.
Our empirical results on a Linux testbed demonstrate the
success of our approach in supporting deferrable servers
and online admission control for both aperiodic and peri-
Proceedings of the 13th IEEE Real Time and
Embedded Technology and Applications Symposium(RTAS'07)
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