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SystemTap (the kernel probing tool build into Linux) is coming along pretty well - I needed to create something to alert me when arbitrary apps set TCP_NODELAY last week, spend a little while looking at SystemTap, made a simple script that probes at tcp.setsockopt(), got the info I needed, and printed it presentably.

I added the script to http://sourceware.org/systemtap/wiki/WarStories . Check it out and others, it's pretty easy to pick up.

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SystemTap is great for general system issues, but isn't nearly as good when trying to look at issues that occur inside the JVM.

True. I believe they're adding providers to the major VMs though (I mainly work in Python so I focus on the cPython VM, but I have heard of some early JVM introspection work too).

PS. Check out http://fedoraproject.org/wiki/Features/SystemtapStaticProbes for info on current Java status (which seems to be more developed than we think).

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