[rfc][patch] Eliminate quadratic slow-down on number of solibs.

Paul Pluzhnikov
2009-04-20T23:29:12Z

Greetings,

We have a test case which uses ~2800 shared libraries.

Running this test with '--version' (i.e. run to main and exit) takes about
40 seconds.

Running it under GDB (CVS Head) with a breakpoint set on main takes 15
minutes, with significant CPU usage by GDB itself. Profiling GDB, I see
that the 2 top contributors are:

time seconds seconds calls s/call s/call name
61.66 728.26 728.26 2794 0.26 0.26 find_methods
24.21 1014.18 285.91 3904625 0.00 0.00 lookup_minimal_symbol_text

The former call comes from breakpoin_re_set(), as GDB scans all libraries
loaded so far, looking to see if any of them define Objective-C methods
which match "main" as a selector.

The latter call comes from repeaded calls to create_overlay_event_breakpoint,
which again scans all libraries loaded so far for "_ovly_debug_event".

AFAICT, both of the above scans are quadratic in number of solibs; and we
don't use either overlays or Objective-C, so this is "pure waste" for us.

To address the repeated scanning for objective-c, I propose adding
n_objc_syms to the struct objstats, initializing that field to ~0,
and counting them in find_methods the first time we encounter a given
objfile. If we see that objfile again, and n_objc_syms == 0, find_methods
could return immediately.

To address the overlay_event_breakpoint, I propose attached patch.
[Tested on Linux/x86_64 with no regressions.]

Comments?

Thanks,
--
Paul Pluzhnikov

ChangeLog:

2009-04-20 Paul Pluzhnikov <ppluzhnikov@google.com>

* breakpoint.h: Add breakpoint_re_set_objfile prototype.

* breakpoint.c (breakpoint_re_set_objfile): Don't rescan
all objfiles unnecessarily.
(breakpoint_re_set): New function.

* symfile.c (new_symfile_objfile): Call breakpoint_re_set_objfile
instead of breakpoint_re_set.

* objfiles.c (objfile_relocate): Likewise.

Index: breakpoint.h
===================================================================
RCS file: /cvs/src/src/gdb/breakpoint.h,v
retrieving revision 1.90
diff -u -p -u -r1.90 breakpoint.h
--- breakpoint.h 31 Mar 2009 16:44:17 -0000 1.90
+++ breakpoint.h 20 Apr 2009 23:02:11 -0000
@@ -687,7 +687,7 @@ extern int breakpoint_thread_match (CORE
extern void until_break_command (char *, int, int);

extern void breakpoint_re_set (void);
-
+extern void breakpoint_re_set_objfile (struct objfile *);
extern void breakpoint_re_set_thread (struct breakpoint *);

extern struct breakpoint *set_momentary_breakpoint
Index: breakpoint.c
===================================================================
RCS file: /cvs/src/src/gdb/breakpoint.c,v
retrieving revision 1.390
diff -u -p -u -r1.390 breakpoint.c
--- breakpoint.c 31 Mar 2009 16:44:17 -0000 1.390
+++ breakpoint.c 20 Apr 2009 23:02:11 -0000
@@ -7711,7 +7711,7 @@ breakpoint_re_set_one (void *bint)

/* Re-set all breakpoints after symbols have been re-loaded. */
void
-breakpoint_re_set (void)
+breakpoint_re_set_objfile (struct objfile *objfile)
{
struct breakpoint *b, *temp;
enum language save_language;
@@ -7730,8 +7730,17 @@ breakpoint_re_set (void)
}
set_language (save_language);
input_radix = save_input_radix;
-
- create_overlay_event_breakpoint ("_ovly_debug_event");
+
+ if (objfile != NULL)
+ create_overlay_event_breakpoint_1 ("_ovly_debug_event", objfile);
+ else
+ create_overlay_event_breakpoint ("_ovly_debug_event");
+}
+
+void
+breakpoint_re_set (void)
+{
+ breakpoint_re_set_objfile (NULL);
}
\f
/* Reset the thread number of this breakpoint:
Index: symfile.c
===================================================================
RCS file: /cvs/src/src/gdb/symfile.c,v
retrieving revision 1.224
diff -u -p -u -r1.224 symfile.c
--- symfile.c 7 Apr 2009 20:43:51 -0000 1.224
+++ symfile.c 20 Apr 2009 23:02:11 -0000
@@ -922,7 +922,7 @@ new_symfile_objfile (struct objfile *obj
}
else
{
- breakpoint_re_set ();
+ breakpoint_re_set_objfile (objfile);
}

/* We're done reading the symbol file; finish off complaints. */
Index: objfiles.c
===================================================================
RCS file: /cvs/src/src/gdb/objfiles.c,v
retrieving revision 1.82
diff -u -p -u -r1.82 objfiles.c
--- objfiles.c 11 Mar 2009 20:26:02 -0000 1.82
+++ objfiles.c 20 Apr 2009 23:02:11 -0000
@@ -674,7 +674,7 @@ objfile_relocate (struct objfile *objfil
}

/* Relocate breakpoints as necessary, after things are relocated. */
- breakpoint_re_set ();
+ breakpoint_re_set_objfile (objfile);
}
\f
/* Many places in gdb want to test just to see if we have any partial