Re: fadvise DONTNEED implementation (or lack thereof)

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> > I've recently been trying to track down the root cause of my server's
> > persistent issue of thrashing horribly after being left inactive. It
> > seems that the issue is likely my nightly backup schedule (using rsync)
> > which traverses my entire 50GB home directory. I was surprised to find
> > that rsync does not use fadvise to notify the kernel of its use-once
> > data usage pattern.
> > 
> > It looks like a patch[1] was written (although never merged, it seems)
> > incorporating fadvise support, but I found its implementation rather
> > odd, using mincore() and FADV_DONTNEED to kick out only regions brought
> > in by rsync. It seemed to me the simpler and more appropriate solution
> > would be to simply flag every touched file with FADV_NOREUSE and let the
> > kernel manage automatically expelling used pages.
> > 
> > After looking deeper into the kernel implementation[2] of fadvise() the
> > reason for using DONTNEED became more apparant. It seems that the kernel
> > implements NOREUSE as a noop. A little googling revealed[3] that I not
> > the first person to encounter this limitation. It looks like a few
> > folks[4] have discussed addressing the issue in the past, but nothing
> > has happened as of 2.6.36. Are there plans to implement this
> > functionality in the near future? It seems like the utility of fadvise
> > is severely limited by lacking support for NOREUSE.
> 
> btw, Other OSs seems to also don't implement it.
> example,

I've heared other OSs status of fadvise() from private mail.

NetBSD: no-op (as linux)
FreeBSD/DragonflyBSD/OpenBSD: don't exist posix_fadvise(2)



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