On Thu, Aug 8, 2013 at 12:25 PM, Andy Lutomirski [off-list ref] wrote:

Whoops -- I read your email too quickly.  I haven't tried
MADV_WILLNEED, but I think I tried reading each page to fault them in.
 Is there any reason to expect MADV_WILLNEED to do any better?  I'll
try to do some new tests to see how well this all works.

(I imagine that freshly fallocated files are somehow different when
read, since there aren't zeros on the disk backing them until they get
written.)

Well, this will teach me to write code based on an old benchmark from
memory.  It seems that prefaulting for read is okay on Linux 3.9 --
latencytop isn't do_wp_page or ext4* at all, at least not for the last
couple minutes on my test box.

I wonder if ext4 changed its handling of fallocated extents somewhere
between 3.5 and 3.9.  In any case, please consider these patches
withdrawn for the time being.

* With file_update_time stubbed out.  I need to dust off my old
patches to fix that part.