On Sun, 2009-11-08 at 16:03 +0100, Lennart Poettering wrote:
> On Fri, 06.11.09 20:22, Alexander Larsson ([email protected]) wrote:
>
> > There is one problem with POSIX_FADV_DONTNEED. If you do it on a file
> > the kernel will drop it from its caches. This is generally what you want
> > if you just indexed a 100 meg text file that no other app cares about
> > atm, since it means we won't throw out 100 megs of otherwise useful
> > caches. However, if you're reading a file that some other app actually
> > cares about this may be a problem, since you're now ensuring that the
> > file has to be re-read the next time that app wants to use the file. Not
> > sure if there is a better way though...
>
> Shoudln't MADV_SEQUENTIAL do this? Enables aggressive read-ahead and
> quick freeing according to the man page. Not sure though if the latter
> is actually implemented by the VM in the way we'd want it here.
I don't see anything in SEQUENTIAL mentioning that you don't want the
file cached. It seems to be mainly about readahead.
This seems like what you'd really want:
POSIX_FADV_NOREUSE
The specified data will be accessed only once.
However, its useless:
In kernels before 2.6.18, POSIX_FADV_NOREUSE had the same
semantics as POSIX_FADV_WILLNEED. This was probably a bug; since
kernel 2.6.18, this flag is a no-op.
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