Re: statistics with intermediate results

[fche at redhat dot com (Frank Ch. Eigler)]

Martin Peschke <mp3@de.ibm.com> writes:

> [...]
> The problem is where to put the first timestamp. It would
> be per request. But when I use dynamic instrumentation, e.g.
> systemtap, then I can't put some spare bytes in a
> per request data structure to store intermediate results.

I don't understand what is blocking you.  There is no "per request
data structure" in systemtap - spare or otherwise.  You copy values
out of kernel side with the $target variables, and correlate them on
the script side.

You can declare and use as many script-side arrays as you see fit, and
index them as you see fit.  As long as you can recompute the same
index tuple (a pid, request pointer address, and/or whatever) at the
probe points that correspond to the beginning and the end of a
computation, just use the array to store the temporaries ("start
time").

Once you have a real result ("elapsed time") you want to store, put
that in a new array, which can be one that carries statistical values.
Use the "<<<" accumulation operator to add values, and the @avg etc.
operators to read results.


> [...] I guess, one could report all events, like send time, receive
> time and so on, through systemtap and defer all processing to a user
> land script. That's the Linux Kernel Event Trace Tool approach:
> [...]

It is a possible way, but not generally necessary for systemtap.


- FChE