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Re: .deb format: let's use 0.939, zstd, drop bzip2




On Fri, May 10, 2019 at 09:15:49PM +0100, Ian Jackson wrote:
> dpkg-deb has a built-in decoder for the subset of ar that is used for
> deb(5).  One reason I chose ar rather than tar is that handwriting a
> decoder for ar was much simpler than for tar.

I wonder then, why the speed loss?  A good ar decoder should be able to peel
away that layer without any difference: you just tell the tar decoder that
it has a tarball X bytes long.  (Ar members are always contiguous without
holes or sub-blocks, right?)

A stock library like libarchive has to be able to handle arbitrary filters,
the vast majority of which are not transparent zero-copy.  Thus, I
understand why my experiments show the difference with a libarchive-based
backend[1].  But if dpkg is faster with 0.939, that's an obvious missed
optimization.


Meow!

[1}. libarchive claims to be zero-copy here but it splits the input into
tiny blocks.
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