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Re: gcc dwarf2out: Drop the size + performance overhead ofDW_AT_sibling


Hi Jan,

On Tue, 2011-10-18 at 11:44 +0200, Jan Kratochvil wrote:
> On Tue, 18 Oct 2011 11:26:03 +0200, Mark Wielaard wrote:
> > On Mon, 2011-10-17 at 15:36 +0200, Jan Kratochvil wrote:
> > > gcc.post: Drop DW_AT_sibling; remove 27 LoC: -3.49% .debug size, -1.7%
> > > GDB time.
> > 
> > Do you have more information about that? Systemtap for example, which
> > uses elfutils libdw uses DW_AT_subling to more efficiently go through
> > the debug_info DIEs.
> 
> The patch with various benchmarks is:
> 	[patch] dwarf2out: Drop the size + performance overhead of DW_AT_sibling
> 	http://gcc.gnu.org/ml/gcc-patches/2011-10/msg00992.html
> 
> GDB also uses DW_AT_sibling when available (skip_one_die and
> locate_pdi_sibling).  The mail above quotation:
> # I guess DW_AT_sibling had real performance gains on CPUs with 1x (=no) clock
> # multipliers.  Nowadays mostly only the data size transferred over FSB matters.
> 
> The problem is the DIEs skipping by CPU is so cheap on current CPUs it cannot
> be compared with the overhead of providing the helper data for it.  I did not
> expect dropping DW_AT_sibling would be even a consumer performance
> _improvement_.  I expected more it will be either not measurable or just not
> significant enough for the .debug on-disk sizes cost justification.
> 
> I did only gdb and idb benchmarks.  systemtap benchmark is welcome, libstdc++
> files for benchmark, if it is enough for systemtap this way:
> 	http://people.redhat.com/jkratoch/ns.tar.xz

Thanks for those. Some quick benchmarks show systemtap selection of
functions and function parameters is slightly slower without
DW_AT_sibling being available. But not dramatically.

$ for i in $(find ns -name libstdc++.so.6.0.17.debug); do echo $i; time
stap -l "process(\"$i\").function(\"*\")" | wc --lines; done
ns/gccgit-c-xxxxxxxxxxxxx-test/default/libstdc++.so.6.0.17.debug
1852

real	0m0.427s
user	0m0.392s
sys	0m0.036s
ns/gccgit-c-xxxxxxxxxxxxx-test/gdbindex/libstdc++.so.6.0.17.debug
1852

real	0m0.422s
user	0m0.384s
sys	0m0.035s
ns/gccgit-c-ns-xxxxxxxxxx-test/default/libstdc++.so.6.0.17.debug
1852

real	0m0.447s
user	0m0.406s
sys	0m0.042s
ns/gccgit-c-ns-xxxxxxxxxx-test/gdbindex/libstdc++.so.6.0.17.debug
1852

real	0m0.443s
user	0m0.404s
sys	0m0.037s

That is selecting all functions in libstdc++. Systemtap doesn't use
gdbindex, but I included it so you can see the "noise". Here the
slowdown seems somewhat equal to the noise.

If we also want parameters/variables listed for each function probe
point (using -L) things are a bit more visible:

$ for i in $(find ns -name libstdc++.so.6.0.17.debug); do echo $i; time
stap -L "process(\"$i\").function(\"*\")" | wc --lines; done
ns/gccgit-c-xxxxxxxxxxxxx-test/default/libstdc++.so.6.0.17.debug
1852

real	0m0.573s
user	0m0.522s
sys	0m0.043s
ns/gccgit-c-xxxxxxxxxxxxx-test/gdbindex/libstdc++.so.6.0.17.debug
1852

real	0m0.558s
user	0m0.505s
sys	0m0.052s
ns/gccgit-c-ns-xxxxxxxxxx-test/default/libstdc++.so.6.0.17.debug
1852

real	0m0.611s
user	0m0.556s
sys	0m0.056s
ns/gccgit-c-ns-xxxxxxxxxx-test/gdbindex/libstdc++.so.6.0.17.debug
1852

real	0m0.603s
user	0m0.557s
sys	0m0.045s

Still no dramatic slowdown, but probably enough to discount random noise
in the measurements.

So DW_AT_sibling definitely does help systemtap/libdw walk a little bit
more efficient through the DIE tree. But you are right that walking the
DIE tree even without them can be done pretty quickly.

Cheers,

Mark


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