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Date: Mon, 26 Mar 2007 06:23:17 -0600
From: Eric Blake <ebb9 AT byu DOT net>
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To: cygwin AT cygwin DOT com
Subject: Re: bash completion slow
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According to Christopher Wingert on 3/25/2007 8:35 AM:
> Hi
> 
> I have a directory where there are about 300 subdirectories in it.  When I
> hit tab, not only is the delay to the "Display all 300 possibilities..."
> is slow but the screw draw after answer y is almost like a 300 baud modem.
>  I've read on these lists about the triple stat issue with cygwin, is this
> the cause of this problem?  Any solution to this problem?

What 'triple stat' issue are you referring to?  Part of the problem is
that tab completion relies on determining file type (directory vs. regular
file, etc.), but unlike Linux, cygwin readdir() does not convey file type
(because it is a relatively expensive operation for some file types, such
as symlinks vs. block and character devices, involving opening the file,
which would penalize readdir for other cases), while stat() conveys more
information than readline cares about (involving even MORE expensive
operations, the result of which is just discarded).  With directories in
particular, part of the slowdown is that stat()'s st_nlink field is
populated by counting how many files appear in that sub-directory, even
though readline didn't care about that.  And part of the problem is that
Windows just STINKS at enumerating directory contents.  Overall, it means
that where there are 300 subdirectories, and readline is stat()ing all 300
of them, cygwin ends up using Windows slow traversal of all 300 directories.

If readdir were to provide d_type in struct dirent, even populating it
only with DT_DIR vs. DT_UNKNOWN, then readline's tab completion and
several of the coreutils would gain some speedups, merely by being able to
distinguish directories from other files without having to stat()
directories.  But SHTDI and write such a patch, and prove that it doesn't
penalize other uses of readdir that could care less about file type.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9 AT byu DOT net
volunteer cygwin readline maintainer
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