Performance Overhead of Perf Event Subsystem in Linux Kernel

Performance counters for Linux are a new kernel-based subsystem that provide a framework for all things开发者_开发问答 performance analysis. It covers hardware level (CPU/PMU, Performance Monitoring Unit) features and software features (software counters, tracepoints) as well. Since 2.6.33, the kernel provide 'perf_event_create_kernel_counter' kernel api for developers to create kernel counter to collect system runtime information. What I concern most is the performance impact on overall system when tracepoint/ftrace is enabled. There are no docs I can find about them. I was once told that ftrace was implemented by dynamically patching code, will it slow the system dramatically?

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Dynamic patching is used as a way to minimize overhead when tracing isn't activated. The overhead of the PMU-based hardware counters should depend on how many different events you're sampling, and how often. It's probably low enough for you to find an acceptable configuration, depending on what you want. I would try enabling whatever information collection you're interested in, carefully set the frequency, and then check empirically whether the overhead is acceptable.

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Those things are put in because people think they might be useful, and no doubt they can collect interesting information. If that is the goal, fine.

However, if the goal is to tune software for higher performance, they are of little diagnostic value.

They can provide interesting measurements, but they don't tell you what to fix.

This method does tell you what to fix.