jsadebugd/jdb and running clear JVM

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Using jsadebugd or jdb it's possible to debug already running JVMs, started without any arguments. I don't understand how it's possible, because I thought, that JVM compiles bytecode in native code by JIT, and it cannot be debuged.

Can anybody explain me ?

And how such time-to-time debug affects on performance?

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Notably slows performance, however only if debugger is attached and that is only possible if compiled to allow debugging (optional).