[Dwarf-Discuss] debug_aranges use and overhead

Greg Clayton clayborg@gmail.com
Fri Mar 19 18:15:02 GMT 2021 


> On Mar 19, 2021, at 9:33 AM, Samy Al Bahra via Dwarf-Discuss <dwarf-discuss at lists.dwarfstd.org> wrote:
> 
> Hi David,
> 
> Sorry I'm a bit late to the game. On the value of having .debug_aranges and the performance impact:
> 
> Our debugger was designed for performance and does end to end lazy evaluation (down to the DIE). This is quite old (excuse the formatting) but numbers are here: https://engineering.backtrace.io/2014-09-15-bt-lightweight-backtrace-tool/ <https://engineering.backtrace.io/2014-09-15-bt-lightweight-backtrace-tool/> , search for "Chromium".  This is something other debuggers can take advantage of if they run in a non-interactive / batch mode (think bulk processing of millions - billions of dumps a month) and is generally useful when folks are iterating in development (fast feedback for crashes while having some background indexing work going on). I'm also happy to run benchmarks for you with and without .debug_aranges on top of our debugger if it'll be useful.
> 
> One of the crucial optimizations we made is incremental indexing on top of .debug_aranges based on PC values (+ complexities Greg mentions later in the thread). In cases where we lack this, we use our own persistent cache which introduces unnecessary complexity. Now I am considering going as far as adding a multi-threaded indexer for cases where a persistent cache / build system modifications aren't an option (work to begin in the next week or two).


For symbolicating in bulk, millions - billions of dumps a month, I would highly suggest you check out converting your DWARF to GSYM. This file format was created because of the inefficiencies of using DWARF for symbolication when done at a large scale on servers. The file format is designed to be mapped into memory and used as is. It is many times smaller than DWARF and the lookup speeds are orders of magnitude faster. The file format is designed to be mmap'ed into one or more processes as shared memory so that if your symbolication servers can route the symbolication requests for the same GSYM file to the same server then pages from that GSYM file can be hot in the file cache and speed up requests event further. 

Features of the GSYM file format:
- Address table is sorted for fastest lookups and doesn't store full 64 bit addresses, just offsets from a base address from the file header. This allows you to search for your address and touch as few pages as possible when doing the lookup. You are also only searching addresses when doing the search, no extra data. The offsets to the data for each address are in a separate table that follows the address table.
- Single uniqued file table to store source file paths for any data that is references within the GSYM file (like the source files for the line tables). This means all compile unit file tables from the DWARF, which each have their own file tables in each prologue of the info in .debug_line, are uniqued into a single file table which reduces GSYM file size. The file table also splits all paths up into directory + basename which allows all files in the same directory to shared the same string in the string table for the directory path.
- All function info is stored in one contiguous blob of data and includes:
  - full line table for the function only (no more parsing the entire line table of a compile unit just to get the lines for a function)
  - inline call stack information to unwind inline call stacks

The GSYM file format description is contained in the header files in the headerdoc. The GSYM file format is open sourced into llvm.org <http://llvm.org/> and the code can be found:

Library code:

  llvm/include/llvm/DebugInfo/GSYM
  llvm/lib/DebugInfo/GSYM

Tool code:

  llvm/tools/llvm-gsymutil


If you build llvm-gsymutil you can convert any DWARF to GSYM using:

  $ llvm-gsymutil --convert a.out -o a.out.gsym

We have seen GSYM files up to 20x smaller than the DWARF file, which to be fair often includes all other sections (.text, .data, etc).

If you want to see exactly what is encoded in the GSYM file you can dump the entire file using:

  $ llvm-gsymutil -o a.out.gsym

This will give you a great idea of exactly what is encoded in the GSYM file as the dump accurately mirrors the exact contents of the file.

Then lookups can be done with a the tool:

  $ llvm-gsymutil --address 0x1000 a.out.gsym

Or by linking against the LLVM library code and doing it with a few lines of code:

  llvm::Expected<GsymReader> Gsym = GsymReader::openFile(GSYMPath);
  if (!Gsym)
    error(GSYMPath, Gsym.takeError());


  if (llvm::Expected<LookupResult> Result = Gsym->lookup(Addr))
    OS << Result.get();
  else
    error(Result.takeError());

The LookupResult object has all of the information including function + source file + source line for all frames from the inline functions and concrete functions from the one address you looked up.

Anyone interested in learning more can feel free to message me directly offline for more information or support on GSYM.

Greg Clayton

> .debug_aranges would provide a lot of value to our users.
> 
> On Thu, Mar 11, 2021 at 3:48 PM David Blaikie via Dwarf-Discuss <dwarf-discuss at lists.dwarfstd.org <mailto:dwarf-discuss at lists.dwarfstd.org>> wrote:
> On Thu, Mar 11, 2021 at 5:48 AM <paul.robinson at sony.com <mailto:paul.robinson at sony.com>> wrote:
> Hopefully not to side-track things too much... maybe wants its own
> thread, if there's more to debate here.
> 
> Yeah, how about we spin it off into another thread (done here)
>  
> >> For the case you suggested where it would be useful to keep the range
> >> list for the CU in the .o file, I think .debug_aranges is what you're
> >> looking for.
> >
> > aranges has been off by default in LLVM for a while - it adds a lot of
> > overhead (doesn't have all the nice rnglist encodings for instance -
> > nor can it use debug_addr, and if it did it'd still be duplicate with
> > the CU ranges wherever they were).
> 
> Did you want to file an issue to improve how .debug_aranges works?
> 
> I don't currently understand the value it provides, and I at least don't have a use case for it, so I'm not sure I'd be the best person to advocate/drive that work.
> 
> Complaining that it duplicates CU ranges is missing the point, though;
> it's an index, like .debug_names, of course it duplicates other info.
> If you want to suggest an improved index, like we did with .debug_names,
> that would be great too.
> 
> .debug_names is quite different though - it collects information from across the DIE tree - information that is expensive to otherwise gather (walking the whole DIE tree).
> 
> .debug_aranges is not like that for most producers (producers that do include the address ranges on the CU DIE) - the data is readily available immediately on the CU. That does involve reading some of .debug_abbrev, and interpreting a handful of attributes - but at least for the use cases I'm aware of, that overhead isn't worth the size increase.
> 
> Do you have numbers on the benefits of .debug_aranges compared to parsing the ranges from CU DIEs?
> 
> (one possible issue: the CU doesn't /have/ to contain low/high/ranges if its children DIEs contain addresses - having that as a guarantee, or some preferred way of encoding zero length (high/low of 0 would be acceptable, I guess) would be nice & make it cheap to skip over CUs that don't have any address ranges)
> 
> Roughly, a modern debug_aranges to me would look something like:
> 
> <length>
> <version>
> <CU sec_offset>
> <addr_base>
> <rnglist sec_offset>
> 
> So it could fully re-use the rnglist encoding. If this was going to be as compact as possible, it'd need to be configurable which encodings it uses - ranges V high/low, addrx V addr - at which point it'd probably look like a small DIE with an inline abbrev (similar to the way DWARFv5 encodes the file and directory entries now, and how debug_names is self-describing) - at which point it looks to me a lot like parsing the CU DIEs.
>  
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> 
> -- 
> Samy Al Bahra [http://repnop.org <http://repnop.org/>]
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