Bug 1401091

Summary: gdb's performance is so terrible that it is unusable
Product: [Fedora] Fedora Reporter: Nicholas Miell <nmiell>
Component: gdbAssignee: Jan Kratochvil <jan.kratochvil>
Status: CLOSED EOL QA Contact: Fedora Extras Quality Assurance <extras-qa>
Severity: unspecified Docs Contact:
Priority: unspecified    
Version: 25CC: fweimer, gbenson, jan.kratochvil, kengert, palves, pmuldoon, sergiodj, thomas, tom
Target Milestone: ---Keywords: Reopened
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Last Closed: 2017-12-12 10:57:39 UTC Type: Bug
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Description Flags
relevant part of perf data recorded during a backtrace command none

Description Nicholas Miell 2016-12-02 19:20:37 UTC
Created attachment 1227464 [details]
relevant part of perf data recorded during a backtrace command

Description of problem:
gdb takes multiple minutes per frame to display a backtrace, the frame command takes multiple minutes to complete, displaying data structures using info locals or info args or the print command takes multiple minutes per element in the structure.

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
$ pgrep firefox
$ gdb
(gdb) set auto-load off 
(gdb) attach 3940
(gdb) bt
(gdb) frame 5
(gdb) info locals

Actual results:
Commands take minutes or tens of minutes to execute.

Expected results:
gdb is a usable product.

Comment 1 Thomas Sondergaard 2017-03-31 14:27:30 UTC
I have the same experience with gdb under fedora 25 when I have debuginfo packages installed. If I'm only using the debugging information from code that I have built on my own, performance is acceptable.

An example running assistant-qt from qt5-assistant-5.7.1-4.fc25.x86_64:

$ dnf install qt5-qtbase-debuginfo
$ gdb -- assistant-qt5
(gdb) b 'CentralWidget::backward()'
(gdb) r
# .. click a link in assistant and click the backward() button
(gdb) bt

The bt command takes 43 seconds to complete.

if I run the exact same command but link with a local developer build of qtbase, like this:

LD_LIBRARY_PATH=/home/ts/src/qtbase-build/lib QT_PLUGIN_PATH=/home/ts/src/qtbase-build/plugins

then the backtrace from 'CentralWidget::backward()' takes less than a second.

Could this be the same issue with dwz compression as was discovered and fixed once already in bug 1149865?

Comment 2 Jan Kratochvil 2017-03-31 14:35:41 UTC
I think this is Bug 1410907 - during F-25 build time .gdb_index was not being generated.

*** This bug has been marked as a duplicate of bug 1410907 ***

Comment 3 Thomas Sondergaard 2017-03-31 15:20:34 UTC
I have rebuilt qt5-qtbase-5.7.1 with rpmbuild from rpm-build- and the performance issue remains.

Comment 4 Jan Kratochvil 2017-03-31 15:22:20 UTC
And does it have the .gdb_index section?  As described in that Bug 1410907.

Comment 5 Pedro Alves 2017-03-31 15:30:39 UTC
Looking at the perf trace attached, makes me suspect something odd with debug info causing GDB to redo an expensive operation over and over:

Does "set opaque-type-resolution off" make any difference?

Comment 6 Thomas Sondergaard 2017-03-31 15:38:14 UTC
(In reply to Jan Kratochvil from comment #4)
> And does it have the .gdb_index section?  As described in that Bug 1410907.


$ readelf -S /usr/lib/debug/usr/lib64/libQt5Core.so.5.7.1.debug |grep gdb_index
  [40] .gdb_index        PROGBITS         0000000000000000  01fb7c94

Comment 7 Thomas Sondergaard 2017-03-31 15:40:37 UTC
(In reply to Pedro Alves from comment #5)
> Looking at the perf trace attached, makes me suspect something odd with
> debug info causing GDB to redo an expensive operation over and over:
> Does "set opaque-type-resolution off" make any difference?

Absolutely. It is very fast with that option turned off. Good find!

Comment 8 Pedro Alves 2017-03-31 15:53:44 UTC
OK, the code in GDB that that option disables reads:

  /* If this is a struct/class/union with no fields, then check
     whether a full definition exists somewhere else.  This is for
     systems where a type definition with no fields is issued for such
     types, instead of identifying them as stub types in the first
     place.  */

  if (TYPE_IS_OPAQUE (type) 
      && opaque_type_resolution 
      && !currently_reading_symtab)
      const char *name = type_name_no_tag (type);
      struct type *newtype;

      if (name == NULL)
	  stub_noname_complaint ();
	  return make_qualified_type (type, instance_flags, NULL);
      newtype = lookup_transparent_type (name);

      if (newtype)
	  /* If the resolved type and the stub are in the same
	     objfile, then replace the stub type with the real deal.
	     But if they're in separate objfiles, leave the stub
	     alone; we'll just look up the transparent type every time
	     we call check_typedef.  We can't create pointers between
	     types allocated to different objfiles, since they may
	     have different lifetimes.  Trying to copy NEWTYPE over to
	     TYPE's objfile is pointless, too, since you'll have to
	     move over any other types NEWTYPE refers to, which could
	     be an unbounded amount of stuff.  */
	  if (TYPE_OBJFILE (newtype) == TYPE_OBJFILE (type))
	    type = make_qualified_type (newtype,
	    type = newtype;

So sounds like we're hitting the scenario described above.  It'd be interesting to know what type this is.

Comment 9 Fedora End Of Life 2017-11-16 18:43:46 UTC
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Comment 10 Fedora End Of Life 2017-12-12 10:57:39 UTC
Fedora 25 changed to end-of-life (EOL) status on 2017-12-12. Fedora 25 is
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