On Thu, Dec 3, 2009 at 10:07 PM, Kevin Fitch <kfitc...@gmail.com> wrote:
> I have a situation that I would like to debug with dtrace, but I am not sure
> how.
>
> I have a process that kicks off, runs for a few seconds (perhaps
> milliseconds) then goes away. And this happens very frequently, think
> old-school CGI based web app. And just for a little more fun, this app makes
> extensive use of shared libraries.
>
> How can I use dtrace? I don't have an existing process to attach the pid
> provider.
>
> I could use the pid$target stuff, but that would linvolve modifying the
> system to kick off the processes with dtrace -c. This would be possible, but
> painful. And, it would mean I would have to use a post-processing step
> should I want/need to aggregate results from multiple processes.
>
You could do what I call the "DTrace two-step": have one DTrace
script catch the exec-success probe firing, predicated on "execname ==
foo", stop the process and kick off a second DTrace script (via
system()) that uses the pid provider. Something like this:
exec-success
/ execname == "myapp" /
{
stop();
system("other-d-script -p %d", pid);
}
This might not do what you want, though, given that you want to
aggregate data from multiple instances of this app inside DTrace.
Another trick you could use is to always start that app via a DTrace
script that specifies the probes of interest. (This script is
essentially a throwaway whose only purpose is to enable these probes.)
You can then have a second DTrace script already running (using the
-Z flag) that wildcards the pid in these probes, e.g.,
"pid*::malloc:entry". The -Z flag will let this script start, even
though it matches nothing. As probes are enabled when the app runs,
the pre-existing wild-carded probe specifications will also match, so
the data collection specified in your second script will occur as you
want.
For example, if you wanted to keep track of the total amount of memory
allocated via malloc(3C) by all the instances of your app, you could
start the app every time with this script using "dtrace -c":
pid$target::malloc:entry
{
/* I don't care about collecting data. */
this->foo = 4;
}
You'd then have the following script already running (don't forget the -Z):
pid*::malloc:entry
/ execname == "myapp" /
{
@a[pid] = sum(arg0);
}
HTH,
Chad
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