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Papers from the Real Time Linux Workshop

Oct 23, 2009, 09:32 (0 Talkback[s])
(Other stories by Jake Edge)

"Finding origins of latencies using Ftrace

"Ftrace developer Steven Rostedt describes how to use ftrace to find unexpected and/or unacceptable latencies, which may be a barrier to realtime processing, in his paper [PDF]. Ftrace is a relatively new tool in the kernel that provides various kinds of tracing information and has some facilities that can be used specifically for tracking down latency issues. Tracers like irqsoff, preemptoff, and wakeup (along with some variants) capture information while the kernel is running in specific modes (i.e. with interrupts disabled, preemption turned off, etc.).

"Rostedt's paper gives a fairly detailed look at the tracers, how to enable them, what they do, and the output they produce. While these latency tracers are active, they capture things like kernel functions called or trace event points encountered while looking for the maximum time spent in the latency-causing modes. By looking at what the kernel is doing when the latency has exceeded expectations, it can lead a developer to the specific cause—which may lead to a way to reduce the latency. Rostedt mentions the JACK "audio connection kit" developers as an early adopter of latency tracing, noting that they found both kernel and JACK bugs that were causing excess latency."

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