Re: Direct io on block device has performance regression on 2.6.x kernel

[Andrew Morton <akpm@osdl.org>]

"Chen, Kenneth W" <kenneth.w.chen@intel.com> wrote:
>
> Losing 6% just from Linux kernel is a huge deal for this type of benchmark.
>  People work for days to implement features which might give sub percentage
>  gain.  Making Software run faster is not easy, but making software run slower
>  apparently is a fairly easy task.
> 
> 

heh

> 
>  > Fine-grained alignment is probably too hard, and it should fall back to
>  > __blockdev_direct_IO().
>  >
>  > Does it do the right thing with a request which is non-page-aligned, but
>  > 512-byte aligned?
>  >
>  > readv and writev?
>  >
> 
>  That's why direct_io_worker() is slower.  It does everything and handles
>  every possible usage scenarios out there.  I hope making the function fatter
>  is not in the plan.

We just cannot make a change like this if it does not support readv and
writev well, and if it does not support down-to-512-byte size and
alignment.  It will break applications.