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Debug info files *.debug contained in *-debuginfo.rpm are very big in general (x86_64 Fedora 32 distribution has debug/ directory of 82GB while all its other files are only 75GB). There exist several methods how to make the *-debuginfo.rpms at least a bit smaller. Fedora 18 started using DWZ tool (from [[Features/DwarfCompressor]]) while [https://gcc.gnu.org/pipermail/gcc-patches/2008-August/246281.html Google implemented] a similar but different approach called -fdebug-types-section.
Debug info files *.debug contained in *-debuginfo.rpm are very big in general (x86_64 Fedora 32 distribution has debug/ directory of 82GB while all its other files are only 75GB). There exist several methods how to make the *-debuginfo.rpms at least a bit smaller. Fedora 18 started using DWZ tool (from [[Features/DwarfCompressor]]) while [https://gcc.gnu.org/pipermail/gcc-patches/2008-August/246281.html Google implemented] a similar but different approach called -fdebug-types-section.


Almost nobody uses existing Fedora DWZ (only Fedora/CentOS/RHEL and SuSE OSes) and so its support is missing in tools like [https://lldb.llvm.org/ LLDB] and [[llvm-dwarfdumphttps://llvm.org/docs/CommandGuide/llvm-dwarfdump.html|llvm-dwarfdump]]. -fdebug-types-section is used internally by Google (produced by clang). Debian does not store any debug info archives. Ubuntu uses neither -fdebug-types-section nor DWZ.
Almost nobody uses existing Fedora DWZ (only Fedora/CentOS/RHEL and SuSE OSes) and so its support is missing in tools like [https://lldb.llvm.org/ LLDB], [[llvm-dwarfdumphttps://llvm.org/docs/CommandGuide/llvm-dwarfdump.html|llvm-dwarfdump]] or binutils readelf. -fdebug-types-section is used internally by Google (produced by clang). Debian does not store any debug info archives. Ubuntu uses neither -fdebug-types-section nor DWZ.


* DWZ advantage: On the whole Fedora distro it saves 3.3% (5GB of the 157GB distribution size)
* DWZ advantage: On the whole Fedora distro it saves 3.3% (5GB of the 157GB distribution size)
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<!-- If you cannot complete your feature by the final development freeze, what is the backup plan?  This might be as simple as "Revert the shipped configuration".  Or it might not (e.g. rebuilding a number of dependent packages).  If you feature is not completed in time we want to assure others that other parts of Fedora will not be in jeopardy.  -->
* Contingency mechanism: Nothing needed, Fedora can continue using DWZ, just some debugging/tracing tools will continue to be incompatible.
* Contingency mechanism: Revert the change in [https://src.fedoraproject.org/rpms/redhat-rpm-config redhat-rpm-config]. Fedora can continue using DWZ, just some debugging/tracing tools will stay incompatible.
<!-- When is the last time the contingency mechanism can be put in place?  This will typically be the beta freeze. -->
<!-- When is the last time the contingency mechanism can be put in place?  This will typically be the beta freeze. -->
* Contingency deadline: beta freeze
* Contingency deadline: beta freeze

Revision as of 14:42, 23 September 2020


Debug Info Standardization

Summary

Fedora 18 implemented Features/DwarfCompressor. As the format did not get widespread and the tool is not much maintained it became burden to make existing debugging tools compatible with Fedora debug info.

Owner

Current status

  • Targeted release: Fedora 34
  • Last updated: 2020-09-23
  • FESCo issue: <will be assigned by the Wrangler>
  • Tracker bug: <will be assigned by the Wrangler>
  • Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

Debug info files *.debug contained in *-debuginfo.rpm are very big in general (x86_64 Fedora 32 distribution has debug/ directory of 82GB while all its other files are only 75GB). There exist several methods how to make the *-debuginfo.rpms at least a bit smaller. Fedora 18 started using DWZ tool (from Features/DwarfCompressor) while Google implemented a similar but different approach called -fdebug-types-section.

Almost nobody uses existing Fedora DWZ (only Fedora/CentOS/RHEL and SuSE OSes) and so its support is missing in tools like LLDB, llvm-dwarfdump or binutils readelf. -fdebug-types-section is used internally by Google (produced by clang). Debian does not store any debug info archives. Ubuntu uses neither -fdebug-types-section nor DWZ.

  • DWZ advantage: On the whole Fedora distro it saves 3.3% (5GB of the 157GB distribution size)
    • If the 3.3% size increase is a concern I can implement a different optimization (talk (2)) as a GCC post-processing phase which would require no changes in any DWARF consumers.
  • DWZ disadvantage: DWZ has currently less support across consumers (LLDB, llvm-dwarfdump, binutils readelf)
  • DWZ disadvantage: DWZ requires 8x times more complicated (LoC count) support in consumers than -fdebug-types-section.
  • DWZ disadvantage: DWZ cannot update LLVM .debug_names index which can be generated only by clang (it cannot be regenerated later for DWZ-compressed file)
  • DWZ disadvantage: DWZ DWARF-5 support is a work-in-progress and there is no plan to support DWARF-5 produced by clang
  • DWZ disadvantage: Compilation (linking) requires 2x as big disk space (as DWZ is processing files after linker and DWZ is incompatible with -fdebug-types-section)
  • DWZ disadvantage: Compilation (linking) is slower

This proposal was originally submitted already for Fedora 18 Features/DebugTypesSections.

Feedback

Benefit to Fedora

Scope

  • Proposal owners:

It affects all packages generating *-debuginfo.rpm.

  • Other developers:

Report any possible debuginfo incompatibility (unexpected).

  • Policies and guidelines:

All the needed changes should be done in redhat-rpm-config. The dwz package can be then retired.

  • Trademark approval: N/A (not needed for this Change)
  • Alignment with Objectives:

The size differences are only for *-debuginfo.rpm which is outside of scope of the listed objectives.

Upgrade/compatibility impact

As *-debuginfo.rpm must exactly match NVRA of its binary package the compatibility is not relevant. Existing tools supporting DWZ will still support the DWZ file format in packages which have not been rebuilt.

How To Test

It is not a big risk, -fdebug-types-section is more widespread and more simple than DWZ. Still I should run: 

* libabigail DWARF comparisons
* testsuites of tools like: clang, lldb, gcc, binutils, gdb, elfutils, rpm

User Experience

  • Better compatibility with existing debugging and tracing tools.
  • Less resource-intensive rebuilds of C++ packages (both in disk space, memory requirements and compilation time).

Dependencies

none

Contingency Plan

  • Contingency mechanism: Revert the change in redhat-rpm-config. Fedora can continue using DWZ, just some debugging/tracing tools will stay incompatible.
  • Contingency deadline: beta freeze
  • Blocks release? No
  • Blocks product? N/A

Documentation

Release Notes

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