[erlang-questions] Fast directory walker

Sergej Jurečko <>
Sat Dec 10 10:05:51 CET 2016


read_file_info does the job of is_dir and file_size in a single call. That was the intention.

Also use file:read_file_info(name,[raw])

Sergej

> On 10 Dec 2016, at 09:42, Benoit Chesneau <> wrote:
> 
> this is kind of bullshit (sorry ;).... at the end this is what does the helpers in filelib:
> https://github.com/erlang/otp/blob/maint/lib/stdlib/src/filelib.erl#L257 <https://github.com/erlang/otp/blob/maint/lib/stdlib/src/filelib.erl#L257>
> 
> except if you have a better algorithm in mind i don't se the point of rewriting something that is aleaready existing ...
> 
> On Sat, 10 Dec 2016 at 09:36, Sergej Jurečko < <mailto:>> wrote:
> Stop using filelib functions. Use file:read_file_info and file:list_dir.
> 
> Sergej
> 
> On Dec 10, 2016 9:29 AM, "Frank Muller" < <mailto:>> wrote:
> Hi Stanislaw
> 
> First, I don't care if I've to use documented/undocumented calls as long as I can achieve my goal: faster dir walking.
> 
> And you're right, here is a detailed comparison with other scripting languages:
> 
> In my /usr/share, there’s:
> 2580 directories
> 28953 files
> 
> 1. Erlang (no io:format/1, just recurse):
> 
> walk(Dir) ->
>     {ok, Files} = file:list_dir(Dir),
>     walk(Dir, Files).
> 
> walk(Dir, [ Basename | Rest ]) ->
>     Path = filename:join([ Dir, Basename ]),
>     case filelib:is_dir(Path) of
>         true  ->
>             walk(Path);
>         false ->
>           %%  io:format("~s~n", [Path]),
>             filelib:file_size(Path)
>     end,
>     walk(Dir, Rest);
> walk(_, []) ->
>     ok.
> 
> timer:tc(fun() -> directoy:walker("/usr/share") end).
> {4662361 <tel:4662361>,ok}
> 
> 2. Python (this code even count the size of dir):
> From: http://stackoverflow.com/questions/1392413/calculating-a-directory-size-using-python <http://stackoverflow.com/questions/1392413/calculating-a-directory-size-using-python>
> 
> import os
> def get_size(start_path = '.'):
>     total_size = 0
>     for dirpath, dirnames, filenames in os.walk(start_path):
>         for f in filenames:
>             fp = os.path.join(dirpath, f)
>             total_size += os.path.getsize(fp)
>     return total_size
> 
> print get_size()
> 
> $ cd /usr/share
> $ time dir_walker.py
> 432034130 <tel:432034130>
> 0.25 real         0.13 user         0.10 sys
> 
> 2. Perl (same, count dir size)
> http://www.perlmonks.org/?node_id=168974 <http://www.perlmonks.org/?node_id=168974>
> 
> use File::Find;           
> my $size = 0;             
> find(sub { $size += -s if -f $_ }, "/usr/share");
> 
> $ time perl dir_walker.pl <http://dir_walker.pl/>
> 432034130 <tel:432034130>
> 0.13 real         0.05 user         0.08 sys
> 
> 3. Ruby (same, count dir size):
> 
> def directory_size(path)
>   path << '/' unless path.end_with?('/')
>   raise RuntimeError, "#{path} is not a directory" unless File.directory?(path)
>   total_size = 0
>   Dir["#{path}**/*"].each do |f|
>     total_size += File.size(f) if File.file?(f) && File.size?(f)
>   end
>   total_size
> end
> puts directory_size '/usr/share’
> 
> $ time walker.rb
> 432028422 <tel:432028422>
> 0.21 real         0.09 user         0.11 sys
> 
> 4. Lua:
> From: http://lua-users.org/wiki/DirTreeIterator <http://lua-users.org/wiki/DirTreeIterator>
> 
> require "lfs"
> 
> function dirtree(dir)
>   assert(dir and dir ~= "", "directory parameter is missing or empty")
>   if string.sub(dir, -1) == "/" then
>     dir=string.sub(dir, 1, -2)
>   end
> 
>   local function yieldtree(dir)
>     for entry in lfs.dir(dir) do
>       if entry ~= "." and entry ~= ".." then
>         entry=dir.."/"..entry
> 	local attr=lfs.attributes(entry)
> 	coroutine.yield(entry,attr)
> 	if attr.mode == "directory" then
> 	  yieldtree(entry)
> 	end
>       end
>     end
>   end
> 
>   return coroutine.wrap(function() yieldtree(dir) end)
> end
> 
> for filename, attr in dirtree("/usr/share") do
>       print(attr.mode, filename)
> end
> 
> $ luarocks install luafilesystem
> $ time lua walker.lua > /dev/null
> 0.30 real         0.16 user         0.14 sys
> 
> Do you need more?
> 
> Thanks for you help.
> /Frank
> 
> Le sam. 10 déc. 2016 à 00:51, Stanislaw Klekot < <mailto:>> a écrit :
> On Fri, Dec 09, 2016 at 11:15:58PM +0000, Frank Muller wrote:
> 
> > I would like to improve the speed of my directory walker.
> 
> >
> 
> > walk(Dir) ->
> 
> >     {ok, Files} = prim_file:list_dir(Dir),
> 
> >     walk(Dir, Files).
> 
> 
> 
> Why prim_file:list_dir() instead of file:list_dir()? The former is
> 
> undocumented internal function.
> 
> 
> 
> [...]
> 
> > Compared to almost anything i found on the web, it’s still very slow:
> 
> > > timer:tc(fun() -> dir:walk("/usr/share") end).
> 
> > {4662361,ok}
> 
> 
> 
> What is it this "anything you found on the web"? And how did you run
> 
> your comparisons? There's a large difference between first and second
> 
> consequent run caused by OS' directory cache, and there's large
> 
> difference between simply walking through the directory and walking with
> 
> printing something to the screen for every file.
> 
> 
> 
> Then there's also your using filelib:is_dir() and then
> 
> filelib:file_size(), which means two stat(2) calls, while you only need
> 
> to do it once per file (file:read_file_info()).
> 
> 
> 
> --
> 
> Stanislaw Klekot
> 
> 
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