[dtrace-discuss] Definition of "anchored" and "unanchored" probes

[Chip Bennett]

These are good thoughts, and maybe I'm "picking nits": its just didn't 
seem clear cut to me.

Perhaps we could round this discussion out with a thought from one of 
the authors of the manual?

Thanks,
Chip

Roch wrote:

>
>Wee Yeh Tan writes:
> > Chip,
> > 
> > I believe the Glossary item "probe" says it the clearest
> >   "....  A probe may be anchored to a particular module and function
> > or it may be unanchored if it is not associated with a particular
> > program location (e.g. profile timer)."
> > 
> > >From the above definition, 'fbt', 'proc' and 'vminfo' probes are
> > definitely anchored.
> > 
> > 'syscall' probes are a little tricky since they are implemented at the
> > gate into the respective OS functions.  The code path for enabled
> > 'syscall' probes goes through dtrace_systrace_syscall(32).  In that
> > sense, I suspect that 'syscall' probes are unanchored but I am really
> > not sure.
> > 
>
>This  possibly  refers to  wether   or  not the  D  variable
>probefunc   and probemod are    meaningful  or not in  probe
>context.  With  syscall   provider  probefunc is,   but  not
>probemod; maybe    that   makes syscalls   probes  partially
>anchored ?
>
>-r
>
> > 
> > -- 
> > Just me,
> > Wire ...
> > 
> > On 7/11/06, Chip Bennett <cbennett at laurustech.com> wrote:
> > > Referring to the DTrace manual:
> > >
> > >     "Module If this probe corresponds to a specific program location,
> > > the name of the module in which the probe is located. This name is
> > > either the name of a kernel module or the name of a user library.
> > >     Function If this probe corresponds to a specific program location,
> > > the name of the program function in which the probe is located."
> > >
> > > and then ...
> > >
> > > "A probe that has a module and function as part of its name is known as
> > > an anchored probe, and one that does not is known as unanchored."
> > >
> > > Here's my question:  By the above definition, "pid" and "fbt" or clearly
> > > anchored providers since their probes specify specific locations in a
> > > program.  However, other providers, like "vminfo" or "proc", while they
> > > obviously fire at a specific location, are not really defined by that
> > > location, but rather by when a certain kind of event occurs.  And then
> > > there's "sysproc" which is probably closer to being more like "fbt", in
> > > the way the user perceives it, than any of the others, but doesn't have
> > > a module, so by the above definition would be unanchored.
> > >
> > > I understand why some providers have modules and some don't (or at least
> > > I think I do):  I'm just looking for clarity on the definition of
> > > anchored and unanchored, so I can explain to others.
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>
>  
>