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Re: next steps


Frank Ch. Eigler <fche@redhat.com> [050926]:
> Hi -
> 
> On Mon, Sep 26, 2005 at 10:11:01AM -0500, Jose R. Santos wrote:
> 
> > [...]  one of the requirements that the tool need is that for every
> > probe inserted, the cpu, pid, tid and a time-stamp (jiffies?) be
> > returned as well as some additional information that would be probe
> > specific.
> 
> function ln /* labeled number */ (name, value) {
>   print (name . "=" . string(value) . " ")
> }
> function trace_common_stuff () {
>   ln ("cpu", cpu()) # cpu() to go into context tapset
>   ln ("pid", pid()) 
>   ln ("jiffies", jiffies()))  # jiffies() to go into in timestamp tapset
> }
> probe THIS, THAT { 
>   trace_common_stuff ()
>   print ("probe specific\n")
> }
> probe OTHER {
>   trace_common_stuff ()
>   print ("other probe specific\n")
> }
> 

Nice. Thanks for the example.

> > While it is possible to grab all these using SystemTap, I wonder if
> > it would make more sense to have a special trace probe that does all
> > this and then have SystemTap exploit this. [...]
> 
> Perhaps the confusion is one of terminology.  A "trace probe" is not a
> thing with an obvious meaning.  Using plain systemtap script pieces
> like the one above can likely get you what you need.
> 
> 
> - FChE

My question really was one of simplicity and performance.  I've looked
at some of the SystemTap generated code and the size of the code
generated is something that could be of concern if I need to have a
system with 20+ different trace events being captured.  What I'm
interested in is if the overhead of using SystemTap to gather this
information will be equal whether you use a script or a special
kprobe.  Overhead is important for the people folks we are currently
designing this tool for.  The previous poster sort of implied that
there were some performance issues with using SystemTap for tracing
things like this.

We will do some performance overhead testing but I would also like to
know if anybody has done this type of measurements already.

Thanks

-JRS


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