From: Ariadne Conill <ariadne@dereferenced.org>
To: ~alpine/devel@lists.alpinelinux.org
Cc: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
Subject: Re: Debug symbol compression
Date: Thu, 02 Jul 2020 11:35:57 -0600
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Hello,

On Thursday, July 2, 2020 11:09:19 AM MDT Alex Xu (Hello71) wrote:
> Modern DWARF debugging sections can be compressed with zlib to save
> storage. This is done using the gcc -gz or objcopy
> --compress-debug-sections options.
> 
> Currently, Alpine does not compress debug sections. I think we should
> consider doing so.
> 
> == Benefits ==
> 
> Compressing debug sections saves a lot of installed disk space. For
> example, I took mesa-dbg (the largest dbg apk currently), ran
> 
>     find . -name \*.debug -exec objcopy --compress-debug-sections {} \;
> 
> and reduced the installed size from 452M to 181M.

This sounds really great, we should do it.

> 
> == Drawbacks ==
> 
> === Compatibility ===
> 
> Some programs may not be compatible with compressed DWARF sections.
> While gdb and lldb support it just fine, libunwind does not support it
> until the not-yet-released v1.5. libexecinfo doesn't support compressed
> DWARF sections, but it doesn't support split debug either, so it's a
> moot point. It also seems that libexecinfo is broken now anyways: [1].

I don't think these are really a problem.  We don't expect reliable symbolic 
backtraces out of these libraries; we just provide them (well, libexecinfo) 
for applications which demand them.

Besides, the worst case is the symbol in the (probably useless) backtrace 
output being "??" instead of the symbol name.  I don't see a huge problem with 
that.

> === Speed ===
> 
> Obviously, compressed debug sections take somewhat longer to load. I
> haven't tested it, but based on my experience using Gentoo, up to a few
> extra seconds loading very large debug symbol files (e.g. mesa, firefox)
> can be expected. This probably also applies to applications taking their
> own backtraces using libunwind, but I'm not sure.

This does not seem like a huge problem either.

> === Size ===
> 
> In theory, compressing debug sections should slightly increase APK size.
> However, in practice, I found this not to be the case. gzip uses a
> fairly small window, so in fact double-compressing large, highly
> compressible files can sometimes reduce the size. In this case,
> musl-dbg tar size (i.e. without apk headers) decreased from 179984559
> bytes to 177951179 bytes. Smaller dbg apks will likely see a slight
> increase.
> 
> Unfortunately, this reverses completely with zstd. Using zstd -9,
> compressing debug sections causes the tar size to increase from
> 134414806 to 178504848. This makes sense, because at high levels, zstd
> compression is much more powerful than gzip. Unfortunately, there seems
> to be no support for zstd or even LZMA DWARF compression at this time,
> so unless we implement something like precomp [2], which seems overly
> complicated for a simple `apk add`, it would increase the resulting apk
> size.

I think the main benefit is on-disk install size.  I am willing to accept a 
little bit of storage increase for -dbg packages if we can reduce their 
install size at large.  We compress man pages for the same reason.

> 
> == Conclusions ==
> 
> Overall, I think that once libunwind is bumped to 1.5, we should start
> compressing debug sections. We shouldn't support old bundled versions of
> gdb or libunwind, and the installed size benefit is substantial. In my
> opinion, the speed difference is negligible compared to the size
> benefits. If we switch to zstd apk format, then that will unfortunately
> likely need to be reverted.

I don't think it will need to be reverted.  We can make it possible to 
override the compression method used on a per-package basis and default -dbg 
packages to using zlib compression, or better yet no compression at all.

Ariadne