Recent Linux [7] kernel introduced an interrupt mitigation method called NAPI to reduce the overhead of interrupt handling procedures caused by frame arrival and transmission. However, NAPI caused serious delay and jitter in such procedures. On the other hand, many conventional real-time communication mechanisms were implemented independently to the existing network stack, and most of such mechanisms disabled the stack. Therefore, it is difficult to utilize both of real-time communication and IP communication in parallel.This paper proposed a parallel implementation method to realize both of real-time communication and IP communication simultaneously in a single computer unit. This method implement real-time network stack and IP network stack in parallel by using multiple queue functionality in a NIC (network interface card). Also, this method applied the interrupt mitigation method only to the IP stack. Therefore, this method achieved to reduce the overhead of the wide bandwidth of IP traffic, also to keep delay and jitter of real-time traffic enough small. Some experiments were conducted to measure the performance of the proposed method. The RTT (round trip time) delay of realtime communication was kept lower than 100 µs even in high traffic load of 800 Mbit/s with small jitter, while the RTT of IP communication varied between 100 µs to 20 ms. This paper introduced the design, implementation, and evaluation of the proposed method.
This paper proposed a precise time synchronization method between schedulers of real‐time operating systems by communicating time stamps over a network. An NTP‐based communication protocol was used to measure the time offset between nodes. To minimize the measurement error, a light‐weight network stack called RT‐Messenger was used instead of common transmission control protocol/Internet protocol (TCP/IP) stack. Also, a Kalman filter was introduced to eliminate such error and to estimate time offset and drift. Furthermore, a real‐time scheduler in RTAI/Linux (where RTAI is real‐time application interface) was modified to compensate the time difference using the estimated offset and drift. The time compensation of a scheduler clock affects the next resume times for real‐time tasks. Therefore, a recalculation process of next resume times for all real‐time tasks and a reordering process of real‐time task queue were added to the real‐time scheduler. To validate the proposed method, time differences of resume times for two real‐time tasks on different nodes were measured under various experimental conditions. The results confirm that the resume time differences were successfully kept lower than 400 ns with 300 ns standard deviation under the condition of observation period at 1 s and correction period at 10 s.
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