fadvise DONTNEED implementation (or lack thereof)

[Ben Gamari]

On Tue,  9 Nov 2010 16:28:02 +0900 (JST), KOSAKI Motohiro <kosaki.motohiro at jp.fujitsu.com> wrote:
> So, I don't think application developers will use fadvise() aggressively
> because we don't have a cross platform agreement of a fadvice behavior.
> 
I strongly disagree. For a long time I have been trying to resolve
interactivity issues caused by my rsync-based backup script. Many kernel
developers have said that there is nothing the kernel can do without
more information from user-space (e.g. cgroups, madvise). While cgroups
help, the fix is round-about at best and requires configuration where
really none should be necessary. The easiest solution for everyone
involved would be for rsync to use FADV_DONTNEED. The behavior doesn't
need to be perfectly consistent between platforms for the flag to be
useful so long as each implementation does something sane to help
use-once access patterns.

People seem to mention frequently that there are no users of
FADV_DONTNEED and therefore we don't need to implement it. It seems like
this is ignoring an obvious catch-22. Currently rsync has no fadvise
support at all, since using[1] the implemented hints to get the desired
effect is far too complicated^M^M^M^Mhacky to be considered
merge-worthy. Considering the number of Google hits returned for
fadvise, I wouldn't be surprised if there were countless other projects
with this same difficulty. We want to be able to tell the kernel about
our useage patterns, but the kernel won't listen.

Cheers,

- Ben

[1] http://insights.oetiker.ch/linux/fadvise.html