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

[Ric Wheeler <rwheeler@redhat.com>]

Andreas Dilger wrote:
> On Sep 23, 2008  10:16 +0100, Alan Cox wrote:
>   
>> On Mon, 22 Sep 2008 20:35:23 -0400
>> "Theodore Ts'o" <tytso@mit.edu> wrote:
>>     
>>> With modern hard drives, reading 64k takes roughly the same time as
>>> reading a 4k block.  So request adjacent inode table blocks to reduce
>>> the time it takes when iterating over directories (especially when doing
>>> this in htree sort order) in a cold cache case.  With this patch, the
>>> time it takes to run "git status" on a kernel tree after flushing the
>>> caches via "echo 3 > /proc/sys/vm/drop_caches", is reduced by 21%.
>>>
>>> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
>>>       
>> Acked-by: Alan Cox <alan@redhat.com>
>>
>> I'm actually suprised that 16 is the magic tuning number you've used and
>> a bigger one isn't even more of a win
>>     
>
> I was going to suggest making this at least a #defined constant instead
> of hard coding the values there.  Making it a mount option and/or /proc
> value would allow further testing.
>
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
>
>   

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.

Ric