On Fri, 09 Feb 2007 23:01:06 +0100 Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > From: Miklos Szeredi <mszeredi@xxxxxxx> > > The time shrink_dcache_parent() takes, grows quadratically with the > depth of the tree under 'parent'. This starts to get noticable at > about 10,000. > > These kinds of depths don't occur normally, and filesystems which > invoke shrink_dcache_parent() via d_invalidate() seem to have other > depth dependent timings, so it's not even easy to expose this problem. > > However with FUSE it's easy to create a deep tree and d_invalidate() > will also get called. This can make a syscall hang for a very long > time. > > This is the original discovery of the problem by Russ Cox: > > http://article.gmane.org/gmane.comp.file-systems.fuse.devel/3826 "The file system mounted on /tmp/z in the example contains 2^50 directories". heh. I do wonder how realistic this problem is in real life. > The following patch fixes the quadratic behavior, by optionally > allowing prune_dcache() to prune ancestors of a dentry in one go, > instead of doing it one at a time. > > Common code in dput() and prune_one_dentry() is extracted into a new > helper function d_kill(). > > shrink_dcache_parent() as well as shrink_dcache_sb() are converted to > use the ancestry-pruner option. Only for shrink_dcache_memory() is > this behavior not desirable, so it keeps using the old algorithm. > I wonder if we should be setting shrink_parents=1 in shrink_dcache_memory()? Because we have this problem where the dentry slabs suffer lots of internal fragmentation and we end up with whole slab pages pinned by a single directory dentry. I expect that if shrink_dcache_memory() were aggressive about reaping newly-childless directory dentries, some improvements might be realised there. If so, we should change prune_dcache() to return the number pruned, so that shrink_dcache_memory() can keep its arithmetic correct. Would require some careful testing and is out of scope for your work. - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
