Re: readahead on directories

[Evgeniy Polyakov]

On Wed, Apr 21, 2010 at 07:51:24PM +0100, Jamie Lokier (jamie@xxxxxxxxxxxxx) wrote:
> Fwiw, I found sorting directories by inode and reading them in that
> order help to reduce seeks, some 10 years ago.  I implemented
> something like 'find' which works like that, keeping a queue of
> directories to read and things to open/stat, ordered by inode number
> seen in d_ino before open/stat and st_ino after.  However it did not
> try to readahead the blocks inside a directory, or sort operations by
> block number.  It reduced some 'find'-like operations to about a
> quarter of the time on cold cache.  I still use that program sometimes
> before "git status" ;-)  Google "treescan" and "lokier" if you're
> interested in trying it (though I use 0.7 which isn't published).

As you might expect it is not really a directory readahead :)
Nad I'm not really sure ext234 can implement it in kernel more optimally
without breaking backward compatibility though.

> > it is not about readdir(). Plain read() is synchronous too. But
> > filesystem can respond to readahead calls and read next block to current
> > one, while it won't do this for next direntry.
> 
> I'm surprised it makes much difference, as directories are usually not
> very large anyway.

Well, having several tens of millions of files in 64k dirs takes from tens of
seconds to minutes to read just because of that.

> But if it does, go on, try FIEMAP and blockdev reading, you know you
> want to :-)

Well, it requires substantial underlying fs knowledge and is not simple
and, well, appropriate to do in some cases.

-- 
	Evgeniy Polyakov
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