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I wonder how they are able to trace and capture variable and stack state at a rate of 20M invocations — per second!?

How is this possible on the JVM?

(I’m aware of the instrumentation API that profilers use but I thought they had to do sampling to limit interference with program performance and they are not capturing all the data req’d for debugging either.)

Anyone know?

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There is no single trick, just lot of hard work.

If there's interest, I can write a blog post about this.

I would love to read this. Thank you.

Pure speculation: You don't need to record everything, only enough information to reconstruct what happened later (e.g. outcomes of branches, results of system calls, etc). You can then run a post-processing phase after execution (or in parallel on another thread) to fill in the extra detail that is needed for efficiently presenting what happened.

Actually, the 20 million events per second is the real event count, and on top of that comes processing..

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