Re: GDB memory usage with compressed debug info

[Mike Gulick <mgulick@mathworks.com>]

On 3/24/21 3:59 PM, Matt Rice wrote:
> On Mon, Mar 22, 2021 at 6:16 PM Christian Biesinger via Gdb <gdb@sourceware.org> wrote:
> 
>> On Thu, Mar 18, 2021 at 7:38 PM Mike Gulick via Gdb <gdb@sourceware.org>
>> wrote:
>>>
>>> On 3/16/21 2:40 PM, Mike Gulick wrote:
>>>> Hi,
>>>>
>>>> I'm observing that GDB memory usage is much higher when I have debug
>>>> info compressed with 'objcopy --compress-debug-sections'.  In a large
>>>> C++ application, I see the instance with uncompressed debug info use
>>>> 46GB VIRTUAL memory and 11GB RSS, and the instance with compressed debug
>>>> info is using 46GB VIRTUAL memory and 42GB RSS.  In case it matters, the
>>>> debug info is separated from the original binary into its own file.
>>>>
>>>> It seems like GDB must load the full uncompressed debug info into memory
>>>> when the underlying files are compressed?
>>>>
>>>> I'm currently using GDB 9.2.  I checked the 10.1 NEWS and didn't see
>>>> anything that looked like it would have changed this result, but I'm
>>>> happy to give it a try if you think it might help.
>>>>
>>>> Is there any chance this could be improved with a patch to GDB, or is
>>>> this just the nature of compressed debug data?
>>>>
>>>> Also, FYI, the NEWS link for the GDB 10.1 release on the GDB home page
>>>> points to the NEWS file for the 9.1 release.
>>>>
>>>> Thanks!
>>>>
>>>> -Mike
>>>
>>> I should add that this high memory usage only occurs when there is a
>>> FILE:LINENUM breakpoint set (or pending).  I can run the application
>>> being debugged, observe that GDB's memory usage is around 11 or 12 GB,
>>> then run 'b foobar.cpp:123', and the GDB memory usage will climb up to
>>> 42 GB.  Interesting that symbolic breakpoints don't trigger this high
>>> memory usage, but line-based breakpoints do.  Does this seem expected?
>>
>> I'm guessing that this is due to parsing full debug info ("expanding
>> psymtabs") which is not necessary for symbolic breakpoints, which can
>> rely on minsyms or maybe psymtabs. Have you tried using a heap
>> profiler to see where the memory usage comes from?
>>
> 
> It could possibly narrow it down if the behavior of FILE:label breakpoints
> differ, though I'm not sure what to expect exactly.
> 

I've never used a heap profiler before, but I tried running a debug 
build of GDB 10.1 with heap profiling via an LD_PRELOAD of the tcmalloc 
library (https://gperftools.github.io/gperftools/heapprofile.html). 
This showed that the stack which was doing the vast majority of the 
allocations was this:

bfd_malloc /gdb-10.1/bfd/libbfd.c:275
bfd_get_full_section_contents /gdb-10.1/bfd/compress.c:320
gdb_bfd_map_section /gdb-10.1/gdb/gdb_bfd.c:724
dwarf2_section_info::read /gdb-10.1/gdb/dwarf2/section.c:161
cutu_reader::cutu_reader /gdb-10.1/gdb/dwarf2/read.c:7339
dw2_get_file_names /gdb-10.1/gdb/dwarf2/read.c:3287
dw2_map_symtabs_matching_filename /gdb-10.1/gdb/dwarf2/read.c:3399
iterate_over_symtabs /gdb-10.1/gdb/symtab.c:558
collect_symtabs_from_filename /gdb-10.1/gdb/linespec.c:3797
symtabs_from_filename /gdb-10.1/gdb/linespec.c:3817
parse_linespec /gdb-10.1/gdb/linespec.c:2613
event_location_to_sals /gdb-10.1/gdb/linespec.c:3150
decode_line_full /gdb-10.1/gdb/linespec.c:3230
parse_breakpoint_sals /gdb-10.1/gdb/breakpoint.c:9037
create_sals_from_location_default /gdb-10.1/gdb/breakpoint.c:13733
bkpt_create_sals_from_location /gdb-10.1/gdb/breakpoint.c:12534
create_breakpoint /gdb-10.1/gdb/breakpoint.c:9253
break_command_1 /gdb-10.1/gdb/breakpoint.c:9411
break_command /gdb-10.1/gdb/breakpoint.c:9482
do_const_cfunc /gdb-10.1/gdb/cli/cli-decode.c:95
cmd_func /gdb-10.1/gdb/cli/cli-decode.c:2181
execute_command /gdb-10.1/gdb/top.c:668
catch_command_errors /gdb-10.1/gdb/main.c:457
captured_main_1 /gdb-10.1/gdb/main.c:1218
captured_main /gdb-10.1/gdb/main.c:1243

I also tried setting a breakpoint using FILE:function style, and the 
memory usage was pretty much equivalent to FILE:LINENUM.  This would 
imply (as would the above stack trace) that matching the file name 
causes the full debug info to be read in.

Thanks,
Mike