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

[Ric Wheeler]

Theodore Tso wrote:
> 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
>   

That sounds about right for modern S-ATA/SAS drives. I would expect that 
having this be a tunable knob might help for some types of storage (SSD 
might not care, but should be faster in any case?).

ric

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