Time Partitioned Slack Demonstration
Increased system-level performance in Integrated Time-and EventTriggered Embedded System Architectures.
Connects real-time embedded computers to the Tactical Internet.
Honeywell is working to solve a number of open processing
problems critical to maintaining our technology advantage in
real-time safety critical systems, such as those found in
avionics and automotive platforms.
Slack stealing efficiently allocates unscheduled and reclaimed
CPU time for aperiodic task execution. Slack servers provide
this rapid response to non-critical aperiodic tasks (when
generalized rate monotonic scheduling (GRMS) is used for
safety critical periodic tasks) by favoring a high-priority,
event-driven aperiodic task when all critical deadlines for
periodic tasks can be guaranteed. Slack Stealing also reclaims
compute time from periodic tasks that complete early for
reassignment to aperiodic tasks. This reclamation capability is
particularly useful in safety-critical systems where worst-case
execution time bounds are often highly conservative.
• Increased system configuration flexibility for reduced
system development and certification costs—slack
server on Honeywell’s Primus DeosTM RTOS cut in
half the “certified” application configurations required
to support customers.
Reduction of hardware requirements—slack stealing optimizes
processor usage, allowing hosting of more applications
without increasing CPU space. Honeywell’s DeosTM
implementation obtained a 7X reduction in reserved processor
utilization for an FTP stack, allowing >90% allocation for
critical periodic tasks, with sufficient slack for aperiodic tasks
to run in unscheduled and reclaimed time. Additionally, an
avionics Communication Management Unit written in SDL
(System Design Language) hosted on a DeosTM
implementation required no additional hardware. The CMU
added to an ARINC 653 architecture did.
• Slack stealing can be implemented on top of COTS
RTOSes—in testing, a thin middle layer
implementation on a COTS RTOS proved fully
functional.
• Better incremental processing performance—improves
display refresh rates, delivering above the guaranteed
minimum whenever processor space is available.
Demonstration Configuration
Slack Stealing efficiently allocates unscheduled and
reclaimed CPU time for aperiodic task execution
Benefits of Slack Stealing Protocol
Time partitioning with slack stealing offers significant benefits
over an ARINC 653 partitioned RTOS.
• More rapid response times for event-triggered
processor tasks layered on time-triggered safety critical
task traffic. Host mixtures of application tasking
models: periodic, event-driven, client/server (e.g., Host
critical vehicle control and C4ISR applications on the
same system).
• Improved network bus integration—slack server
implemented in Honeywell’s DeosTM RTOS yielded
>3X improvement in bus throughput for a COTS FTP
protocol.
Event triggered aperiodic messages ping-pong between a PC
and two embedded target VME/VMIC boards, each co-hosting
a time triggered, safety-critical workload and a COTS IP/UDP
ack/nak message stream for display update flow control.
Applications of Slack Stealing Protocol
• Tactical Internet access for helicopters.
• Integration with SDL applications.
• Valuable “plug and play” enabler technology—
configuration flexibility provided by slack stealing
makes the protocol valuable in modular applications.
• Performance degradations from slack overheads are
often offset for interactive applications with short
exchanges.
Slack stealing for event driven processes has been
demonstrated in MetaH, the foundation for AADL (an
emerging SAE standard). Adaptations for dynamic time
partitions have been implemented and flown in Honeywell’s
Primus Epic® RTOS DeosTM .
The figures below illustrate noteworthy performance benefits
from slack stealing for applications that wait for responses,
even with moderately high periodic utilization (90%) and no
slack reclamation. The bus communication throughput
between distributed hand-shaking protocols is increased
significantly by the protocol’s rapid acknowledgements.
ARINC 653
refresh
position
Slack refresh
position
Helicopter
trail path
Slack stealing expedites aperiodic task processing within the
context of scheduling for critical periodic tasks as evidenced in
the much higher refresh rate of the slack RTOS. Slack stealing
can result in significantly improved system performance—
higher throughput, and faster response times.
Evaluation/Maturity
Honeywell has redefined and extended slack stealing to
support advanced time partitioning concepts such as
selectively sharing slack both within and between time
partitions. Additional extensions include support for:
• dynamic tasks
• dynamic time partitions
• incremental and design-to-time tasks
• overhead accounting policies
• slack priority semantics for non-monotonic task
criticality assignments
For further information contact:
Dr. Pam Binns
Email: pam.binns@honeywell.com
Honeywell Laboratories
3660 Technology Drive
Minneapolis, MN 55418
Response times (bottom) and throughput (top) for three
servers: slack (orange), background (green), and polling
(blue—utilizations are for slack~0.99, for polling ~0.94,
background 0.92. Periodic utilization = 0.90.
This work has been supported by DARPA (DSSA EDCS and
DASADA—DAAH01-00-C-R226, contracts), Army AMCOM,and
Honeywell Laboratories