ls -l takes a forever to finish.

[cpghost]

On Thu, 29 Nov 2007 08:42:44 -0500
Bill Moran <wmoran at potentialtech.com> wrote:

> In response to Wojciech Puchar <wojtek at wojtek.tensor.gdynia.pl>:
> 
> > > ls | wc
> > 
> > strange. i did
> > 
> > [wojtek at wojtek ~/b]$ a=0;while [ $a -lt 10000 ];do mkdir
> > $a;a=$[a+1];done
> > 
> > completed <25 seconds on 1Ghz CPU
> > 
> > ls takes 0.1 seconds user time, ls -l takes 0.3 second user time.
> > 
> > unless you have 486/33 or slower system there is something wrong.
> 
> Another possible scenario is that the directory is badly fragmented.
> Unless something has changed since I last researched this (which is
> possible) FreeBSD doesn't manage directory fragmentation during use.
> If you're constantly adding and removing files, it's possible that
> the directory entry is such a mess that it takes ls a long time to
> process it.

Yes, that's also possible. But sorting is really the culprit here:
it *is* possible to create a directory with filenames in such a way
that it triggers Quicksort's O(N^2) worst case instead of O(N log N).

The following Python (2.5) program calls "ls -lf" and sorts its output
with Python's own stable sort() routine (which is NOT qsort(3)). On a
directory with 44,000 entries, it runs orders of magnitude faster than
"ls -l", even though it has to use the decorate-sort-undecorate idiom
to sort the output according according the filename, and it is
interpreted rather than compiled!

I guess that replacing qsort(3) in
/usr/src/lib/libc/gen/fts.c:fts_sort()
with another sort algorithm which doesn't
expose this anomaly would solve that problem.

--------------------- cut here ------------------ cut here ------------

#!/usr/bin/env python
# sortls.py -- sort output of ls -lf with python's stable sort routine.

import os

def sort_ls_lf(path):
    "Sort the output of ls -lf path"
    os.chdir(path)
    lines = os.popen("ls -lf", "r").readlines()
    dsu = [ (line.split()[-1], line) for line in lines ]
    dsu.sort()
    return ''.join(tupl[1] for tupl in dsu)

if __name__ == '__main__':
    import sys
    if len(sys.argv) < 2:
        print >>sys.stderr, "Usage:", sys.argv[0], "path"
        sys.exit(1)
    path = sys.argv[1]
    
    try:
        print sort_ls_lf(path)
    except IOError:
        pass   # silently absorb broken pipe and other errors

--------------------- cut here ------------------ cut here ------------

Regards,
-cpghost.

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