bug? find_branches or find_bzrdir tries to open all files as bzr locations

[Jelmer Vernooij]

On Wed, 2011-05-18 at 01:02 +0300, Alexander Belchenko wrote:
> 17.05.2011 23:53, Aaron Bentley пишет:
> > On 11-05-16 04:48 AM, Alexander Belchenko wrote:
> >> `bzr qlog` without any parameters run inside shared repository is trying
> >> to detect all available branches and show log for all of them. And often
> >> it has very slow start with many branches with many files.
> > 
> >> So, I think it could perform better for local filesystem if it will skip
> >> plain files. Perhaps symlinks should be inspected as well as
> >> directories, I don't know.
> > 
> > I'm going to assume two things:
> > 1. when we list files, we don't get their filetype
> > 2. statting a file to determine its filetype takes roughly the same
> > amount of time as a failed attempt to open ".bzr/branch-format" where
> > ".bzr" is a file.
> > 
> > If those assumptions are true, the fastest thing we can do is attempt to
> > open .bzr/branch-format.  Testing the filetype would introduce and extra
> > filesystem call, which would be slower.
> 
> If I understand correctly, when bzr tries to find branch then it tries
> to open location with all registered (supported) branch formats.
> So, if first attempt (with first or default registered format) to open
> branch has failed then bzr tries to open the location with next
> format, right? Plus foreign-vcs plugins add more branch formats that
> also should be checked.
> 
> If this is correct then for every directory/files/symlink bzr will try
> to open branch several times, therefore doing many syscalls every
> time. Is it correct?
Bzr only tries to open the .bzr/branchformat file and if it can, it
reads the contents and looks them up in the BranchFormat registry. So a
new bzr branch format does not in any way add to the performance of
find_branches.

> As I can see there are 6 branch formats registered in bzr 2.3 codebase:
> 
> BranchFormat.register_format(__format5)
> BranchFormat.register_format(BranchReferenceFormat())
> BranchFormat.register_format(__format6)
> BranchFormat.register_format(__format7)
> BranchFormat.register_format(__format8)
> BranchFormat.set_default_format(__format7)
> _legacy_formats = [BzrBranchFormat4(),
>     ]
> network_format_registry.register(
>     _legacy_formats[0].network_name(), _legacy_formats[0].__class__)
The only thing that's relevant is the number of probers that's
registered in bzrlib.controldir. bzr itself registers a single prober
for .bzr/branch-format and a single prober for opening remote branches.
The foreign format plugins also each register a single prober.

Cheers,

Jelmer
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