Re: [PATCH 2/3] ext4 directory index: read-ahead blocks v2

[Ted Ts'o <tytso@mit.edu>]

On Mon, Jun 20, 2011 at 10:28:54PM +0200, Bernd Schubert wrote:
> From: Bernd Schubert <bernd.schubert@fastmail.fm>
> 
> changes from v1 -> v2:
> Limit the number of read-ahead blocks as suggested by Andreas.
> 
> While creating files in large directories we noticed an endless number
> of 4K reads. And those reads very much reduced file creation numbers
> as shown by bonnie. While we would expect about 2000 creates/s, we
> only got about 25 creates/s. Running the benchmarks for a long time
> improved the numbers, but not above 200 creates/s.
> It turned out those reads came from directory index block reads
> and probably the bh cache never cached all dx blocks. Given by
> the high number of directories we have (8192) and number of files required
> to trigger the issue (16 million), rather probably bh cached dx blocks
> got lost in favour of other less important blocks.
> The patch below implements a read-ahead for *all* dx blocks of a directory
> if a single dx block is missing in the cache. That also helps the LRU
> to cache important dx blocks.

If you have 8192 directories, and about 16 million files, that means
you have about 2,000 files per directory.  I'll assume that each file
averages 8-12 characters per file, so you need 24 bytes per directory
entry.  If we assume that each leaf block is about 2/3rds full, you
have about 17 leaf blocks, which means we're only talking about one
extent index block per directory.   Does that sound about right?

Even if I'm underestimating the number size of your index blocks, the
real problem you have a very inefficient system; probably something
like 80% or more of the space in your 8192 index blocks (one per
directory) are are empty.  Given that, it's no wonder the index blocks
are getting pushed out of memory.  If you reduce the number of
directories that you have, say by a factor of 4 so that you only have
2048 directories, you will still only have about one index block per
directory, but it will be much fuller, and those index blocks will be
hit 4 times more often, which probably makes them more likely that
they stay in memory.  It also means that instead of pinning about 32
megabytes of memory for all of your index blocks, you'll only pin
about 8 megabytes of memory.

It also makes me wonder why your patch is helping you.  If there's
only one index block per directory, then there's no readahead to
accomplish.  So maybe I'm underestimating how many leaf blocks you
have in an average directory.  But the file names would have to be
very, very, VERY large in order to cause us to have more than a single
index block.  

OK, so what am I missing?

						- Ted