GDB and SystemTap integration improving linker-debugger interface
Table of Contents
It is really nice to see something you did in a project influence in future features and developments. I always feel happy and proud when I notice such scenarios happening, and this time was no different. Gary Benson, a colleague at Red Hat who works in the GDB team as well, has implemented a way of improving the interface between the linker and the debugger, and one of the things he used to achieve this is the GDB <-> SystemTap integration that I implemented with Tom Tromey 2 years ago. Neat!
The problem
You can read a detailed description of the problem in the message Gary sent to the gdb-patches mailing list, but to summarize: GDB needs to interface with the linker in order to identify which shared libraries were loaded during the inferior’s (i.e., program being debugged) life.
Nowadays, what GDB does is to put a breakpoint in _dl_debug_state,
which is an empty function called by the linker every time a shared
library is loaded (the linker calls it twice, once before modifying the
list of loaded shlibs, and once after). But GDB has no way to know what
has changed in the list of loaded shlibs, and therefore it needs to load
the entire list every time something happens. You can imagine how bad
this is for performance…
The solution
What Gary did was to put SDT probes strategically on the linker, so that
GDB could make use of them when examining for changes in the list of
loaded shlibs. It improves performance a lot, because now GDB doesn’t
need to stop twice every time a shlib is loaded (it just needs to do
that when stop-on-solib-events is set); it just needs to stop at the
right probe, which will inform the address of the link-map entry of the
first newly added library. It means GDB also won’t need to walk through
the list of shlibs and identify what has changed: you get that for free
by examining the probe’s argument.
Gary also mentions a discrepancy that happened on Solaris libc, which has also been solved by his patch.
And now, the most impressing thing: the numbers! Take a look at this table, which displays the huge improvement in the performance when using lots of shlibs (the time is in seconds):
| Number of shlibs | 128 | 256 | 512 | 1024 | 2048 | 4096 |
|---|---|---|---|---|---|---|
| Old interface | > 0 | > 1 | > 4 | > 12 | > 47 | > 185 |
| New interface | > 0 | > 0 | > 2 | > 4 | > 10 | > 36 |
Impressive, isn’t it?
Conclusion
This is one the things I like most in Free Software projects: the possibility of extending and improving things by using what others did before. When I hacked GDB to implement the integration between itself and SystemTap, I had absolutely no idea that this could be used for improving the interface between the linker and the debugger (though I am almost sure that Tom was already thinking ahead!). And I can say it is a pleasure and I feel proud when I see such things happening. It just makes me feel more and more certain that Free Software is the way to go :-).