[patch 0/2] Accelerate symbol lookups 15x

Jan Kratochvil
2014-10-20T21:44:17Z

Hi,

this patchset has been developed+tested only on top of
[patchv2] Fix 100x slowdown regression on DWZ files
https://sourceware.org/ml/gdb-patches/2014-10/msg00031.html

While this patchset is technically independent the GDB performance and memory
requirements would not make testing of this patchset possible.

Originally I expected with the patch above (fixing backtrace performance) even
the sluggish interactive GDB performance would get fixed. It was not fixed.
During debugging I get 10-30 seconds for a response to simple commands like:
(gdb) print vectorvar.size()

With this patch the performance gets to 1-2 seconds which is somehow
acceptable. The problem is that dwarf2_gdb_index_functions.lookup_symbol
(quick_symbol_functions::lookup_symbol) may return (and returns) NULL even for
symbols which are present in .gdb_index but which can be found in already
expanded symtab. But searching in the already expanded symtabs is just too
slow when there are 400000+ expanded symtabs. There would be needed some
single global hash table for each objfile so that one does not have to iterate
all symtabs. Which .gdb_index could perfectly serve for, just its
lookup_symbol() would need to return authoritative yes/no answers.

Even after such fix these two simple patches are useful for example for
non-.gdb_index files.

One can reproduce the slugging interactive GDB performance with:
#include <string>
using namespace std;
string var;
class C {
public:
void m() {}
};
int main() {
C c;
c.m();
return 0;
}
g++ -o slow slow.C -Wall -g $(pkg-config --libs gtkmm-3.0)
gdb ./slow -ex 'b C::m' -ex 'maintenance set per-command space' -ex 'maintenance set per-command symtab' -ex 'maintenance set per-command time' -ex r
[...]
(gdb) p <tab><tab>
Display all 183904 possibilities? (y or n) n
(gdb) p/r var
$1 = {static npos = <optimized out>, _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x3a4db073d8 <std::string::_Rep::_S_empty_rep_storage+24> ""}}
Command execution time: 20.023000 (cpu), 20.118665 (wall)
^^^^^^^^^
Space used: 927997952 (+0 for this command)
#symtabs: 186099 (+0), #primary symtabs: 21573 (+0), #blocks: 290353 (+0)

For a non-trivial application with all symtabs expanded it takes 113 seconds.

Benchmark on non-trivial application without 'p <tab><tab>':
Command execution time: 0.496000 (cpu), 0.496882 (wall) --- both fixes
Command execution time: 0.899000 (cpu), 0.908062 (wall) --- just lookup_symbol_aux_objfile fix
Command execution time: 3.492000 (cpu), 3.491791 (wall) --- FSF GDB HEAD

Benchmark on non-trivial application with 'p <tab><tab>':
Command execution time: 7.373000 (cpu), 7.395095 (wall) --- both fixes
Command execution time: 13.572000 (cpu), 13.592689 (wall) --- just lookup_symbol_aux_objfile fix
Command execution time: 113.036000 (cpu), 113.067995 (wall) --- FSF GDB HEAD

No regressions on {x86_64,x86_64-m32,i686}-fedora21pre-linux-gnu in standard
and .gdb_index-enabled runs. Neither of the patches should cause any visible
behavior change.

Jan