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Re: user mode backtrace


Hi -

On Thu, Oct 19, 2006 at 06:15:33PM -0600, David Boreham wrote:

> >[...] one where the compiled systemtap probes get some extract of
> >backtrace-enabling unwind information [...]; and another one where
> >the probe may snapshot only an approximation [...]  and rely on a
> >user-space helper to correct/complete it [...]

> Given the choice I'd prefer the latter because it'd be less intrusive.

Actually, intrusiveness is probably not the real trade-off point.
Both approaches would require scanning the user-space stack.  The
former (the probe knowing unwind info) might even be lighter, in that
it would know which portions of the stack need to be sampled.  The
latter may need to be conservative and transcribe a larger number of
words.  With respect to speed, unwinding vs. conservative sampling
should not be too different.

The trade-offs are probably closer to issues such as expressiveness,
module size, complexity of kernel/user interaction.

- FChE


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