Real­Time USB Communication in the Quest Operating System
Eric Missimer, Ye Li, Richard West
USB Scheduling Problem
CAN bus style, high bandwidth, interconnect for
distributed embedded systems
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Future cyber-physical applications
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Objectives
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Predictable data stream processing from different
devices, e.g. different cameras
QoS guarantees on communication channels sharing
physical network links
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Request 1
2
Given a set of tuples: (w1, t1), (w2, t2), ..., (wn, tn)
wi - time to send transaction i, depends on packet size and type of transaction
ti - interval of transaction i, can either be: 1, 2, 4, 8, 16, 32, ...,1024
Request 2
3
Request 3
4
Request 4
(All Interval = 2)
5
Transactions cannot cross micro-frame (125µs) boundaries
Reordering
Point
6
Given a set of tuples is there an assignment such that all transactions obey their
interval and do not cross micro-frame boundaries
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1
Microframe #
Motivations
BU Operating Systems and Services
0µs
Time in Microframe
125µs
Figure 2: Dynamic Reordering Example
Quest heuristic: sort ti by smallest to largest breaking ties using largest w i first
Experimental Evaluation
Applications
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Automotive systems
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Avionics
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Robotics
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Cyber-physical systems
Figure 7: Bandwidth Allocation
Experimental Setup
Figure 1: RacerX Buggy
Figure 3: Maximum Throughput
Figure 4: Dynamic Reordering
of Transactions
Main Contributions
OS
Quest
Linux
Guarantee bandwidth allocations when opening
connections with communication endpoints
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Table 1: Bandwidth Allocation Results
Dynamically reorder transfer requests to avoid
unnecessary rejections along with a USB scheduling
algorithm that outperforms implementations such as
those found in Linux
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Provide real-time guarantees for both asynchronous
and periodic transfers
References
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Tokens
Third
Tokens
Passed
Beagleboard
Passed With
Without
Isoc
Third
Third
Endpoints
Beagleboard
Beagleboard
Opened
1499
1499
5
1499
300
7
Figure 6: Quest vs. Linux
(First-Fit) USB Scheduling
Figure 5: Real-Time
Asynchronous Transactions
http://www.cs.bu.edu/~richwest/quest.html
Eric Missimer, Ye Li and Richard West, "Real-Time USB Communication in the
Quest Operating System", in Proceedings of the 19th IEEE Real-Time and Embedded
Technology and Applications Symposium (RTAS), Philadelphia, USA, April 2013