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Re: fts: avoid O(n^2) ext3/ext4 performance hit on million+-entry dirs


From: Bruno Haible
Subject: Re: fts: avoid O(n^2) ext3/ext4 performance hit on million+-entry dirs
Date: Thu, 25 Sep 2008 19:52:46 +0200
User-agent: KMail/1.5.4

Jim Meyering wrote:
> +     if (nitems > _FTS_INODE_SORT_DIR_ENTRIES_THRESHOLD
> +         && !sp->fts_compar
> +         && dirent_inode_sort_may_be_useful (sp)) {
> +             sp->fts_compar = fts_compare_ino;
> +             head = fts_sort (sp, head, nitems);
> +             sp->fts_compar = NULL;
> +     }

This uses fts_sort, which uses qsort, which is O(n^2) in the worst case.
(Even the implementation in glibc can be O(n^2), if it guesses that mergesort
takes too much memory.)

Couldn't this be avoided by using Paul's mpsort module, and changing fts_sort
to allocate 50% more room, as scratch space for mpsort?

Bruno





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