[GDB 7.6/GCC 4.8.0] Slowdown in GDB macro processing for cores?

Paul Smith psmith@gnu.org
Wed May 22 18:50:00 GMT 2013


On Fri, 2013-05-17 at 11:15 +0100, Pedro Alves wrote:
> > Against a core file it takes a second or longer per iteration of this
> > loop!  FWIW, the class this macro operates on is very large and contains
> > a lot of data per object.
> > 
> > Any ideas?  Is there a handy way to tell where the slowdown is happening
> > here?  
> 
> I'm really surprised.  If anything, I'd expect cores to be marginally faster.
> I suggest git bisecting gdb looking.  Maybe you'll find the culprit
> easily that way.

Sorry for the delay.  Getting a repro case was harder than it seemed
initially... just running "kill -9" on the process wasn't enough, or
didn't crash at the right spot, or something.  We had to induce a
failure through a particular assert().

However, now I have a core file and executable built with my local copy
of GDB 4.8.0.  If I run the macro using a locally-built copy of GDB
7.5.1, then I get this behavior:

  $ time gdb-7.5.1 -c core.30095 -x macros.gdb myprog \
      -ex "frame 4" -ex "ptracelog master->traceLog" -ex "q"
   ...prints 200 trace log lines...
  real    0m2.811s
  user    0m2.768s
  sys     0m0.028s

so that's good stuff.

Now I try it with GDB 7.6, compiled locally using the identical
configure/etc. commands, and I get this behavior:

  $ time gdb-7.6 -c core.30095 -x macros.gdb myprog \
      -ex "frame 4" -ex "ptracelog master->traceLog" -ex "q"
   ...prints 200 trace log lines...
  real    11m51.113s
  user    11m46.324s
  sys     0m2.168s

That's... not as good :-p.

I've just got this case working now, so I'll look into what's going on
here more.



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