Re: [00/41] Large Blocksize Support V7 (adds memmap support)

[Christoph Lameter <clameter@sgi.com>]

On Wed, 12 Sep 2007, Andrea Arcangeli wrote:

> On Tue, Sep 11, 2007 at 01:41:08PM -0700, Christoph Lameter wrote:
> > The advantages of this approach over Andreas is basically that the 4k 
> > filesystems still can be used as is. 4k is useful for binaries and for 
> 
> If you mean that with my approach you can't use a 4k filesystem as is,
> that's not correct. I even run the (admittedly premature but
> promising) benchmarks on my patch on a 4k blocksized
> filesystem... Guess what, you can even still mount a 1k fs on a 2.6
> kernel.

Right you can use a 4k filesystem. The 4k blocks are buffers in a larger 
page then.
 
> The main advantage I can see in your patch is that distributions won't
> need to ship a 64k PAGE_SIZE kernel rpm (but your single rpm will be
> slower).

I would think that your approach would be slower since you always have to 
populate 1 << N ptes when mmapping a file? Plus there is a lot of wastage 
of memory because even a file with one character needs an order N page? So 
there are less pages available for the same workload.

Then you are breaking mmap assumptions of applications becaused the order 
N kernel will no longer be able to map 4k pages.  You likely need a new 
binary format that has pages correctly aligned. I know that we would need 
one on IA64 if we go beyond the established page sizes.