Re: [PATCH, RFC] ext4: Use preallocation when reading from the inode table

[Theodore Tso <tytso@mit.edu>]

On Tue, Sep 23, 2008 at 08:18:54AM -0400, Ric Wheeler wrote:
> I think that Alan is probably right - the magic number for modern drives  
> is probably closer to 256K. Having it be a /sys tunable (with a larger  
> default) would be a nice way to verify this.

I've played with this a bit, and with the "git status" workload,
increasing the magic number beyond 16 (64k) doesn't actually help,
because the number of inodes we need to touch wasn't big enough.

So I switched to a different workload, which ran "find /path -size 0
-print" with a much larger directory hierarchy.  With that workload I
got the following results:

ra_bits	ra_blocks  ra_kb  seconds  % improvement
0	   1	     4	  53.3		 -
1	   2	     8	  47.3		11.3%
2	   4	    16	  41.7		21.8%
3	   8	    32	  37.5		29.6%
4	  16	    64	  34.4		35.5%
5	  32	   128	  32		40.0%
6	  64	   256	  30.7		42.4%
7	 128	   512	  28.8		46.0%
8	 256	  1024	  28.3		46.9%
9	 512	  2048	  27.5		48.4%

Given these numbers, I'm using a default of inode_readahead_bits of 5
(i.3., 32 blocks, or 128k for 4k blocksize filesystems).  For a
workload that is 100% stat-based, without any I/O, it is possible to
get better results by using a higher number, yes, but I'm concerned
that a larger readahead may end up interfering with other reads.  We
need to run some other workloads to be sure a larger number won't
cause problems before we go more aggressive on this parameter.

I'll send the revised patch in another message.

					- Ted