Abstract
Real-time systems have always been difficult to monitor and debug because of the timing constraints which rule out any tool significantly impacting the system latency and performance. Tracing is often the most reliable tool available for studying real-time systems. The real-time behavior of Linux systems has improved recently and it is possible to have latencies in the low microsecond range. Therefore, tracers must ensure that their overhead is within that range and predictable and scales well to multiple cores. The LTTng 2.0 tools have been optimized for multicore performance, scalability, and flexibility. We used and extended the real-time verification tool rteval to study the impact of LTTng on the maximum latency on hard real-time applications. We introduced a new real-time analysis tool to establish the baseline of real-time system performance and then to measure the impact added by tracing the kernel and userspace (UST) with LTTng. We then identified latency problems and accordingly modified LTTng-UST and the procedure to isolate the shielded real-time cores from the RCU interprocess synchronization routines. This work resulted in extended tools to measure the real-time properties of multicore Linux systems, a characterization of the impact of LTTng kernel and UST tracing tools, and improvements to LTTng.
Highlights
Tracing is a method to study the runtime behavior of a program’s execution
We present the performance of Linux Trace Toolkit next generation (LTTng) in our test environment
To isolate the effect of different latency sources, the tool can optionally first set up an ideal environment by disabling the interrupt requests (IRQs)
Summary
Tracing is a method to study the runtime behavior of a program’s execution It consists in recording timestamped events at key points of the execution. To be suitable for real-time system instrumentation, a tracer must have low-overhead and consistent maximum latency in order to minimize execution timing changes and maintain determinism. The Linux Trace Toolkit generation (LTTng) is a high performance tracer optimized for Linux. It supports both kernel and userspace tracing with coherent timestamps, which allow observing system-wide execution. Our contribution consists in a methodology to measure LTTng-UST tracepoint latency characteristics in a real-time environment, the npt open source tool, and modifications to LTTng and CPU shielding configuration to improve its real-time behavior. We proposed and applied modifications to LTTng-UST in order to lower maximum latency and evaluate its effectiveness
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