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

[Ric Wheeler]

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@xxxxxxx> 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@xxxxxxx>
> > >       
> > Acked-by: Alan Cox <alan@xxxxxxxxxx>
> >
> > 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

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