[MPlayer-dev-eng] [PATCH] Use posix_fadvise in stream_file if available

[Reimar Döffinger]

On Sat, Nov 07, 2009 at 09:22:38PM +0200, Uoti Urpala wrote:
> On Sat, 2009-11-07 at 20:06 +0100, Reimar Döffinger wrote:
> > Also I don't think this is supposed to be s->pos, like this you'll
> > advise the kernel to read ahead the data it just read while almost
> > certainly it would make more sense to tell the kernel to drop that part.
> 
> > posix_fadvise(s->fd, pos, max_len, POSIX_FADV_DONTNEED);
> 
> I think that's a bad idea. At least on Linux POSIX_FADV_DONTNEED seems
> to make the kernel always drop the data from the cache instantly, even
> if there is no memory pressure. So this would require filesystem reads
> for every seek. It'd also completely prevent the file from being in
> cache after playback, and even drop it from the cache if it was already
> there.
> 
> If the behavior of DONTNEED was only "schedule this to be dropped from
> cache first in case of memory pressure" then this could be beneficial.
> But currently it isn't.

If it doesn't cause any serious issues I don't think implementation
idiocies in the current Linux kernel are a good reason to make things
harder for (possibly future) better implementations.
Though probably adding POSIX_FADV_NOREUSE is the better solution,
for a "perfect" implementation it will have the same effect with less
code and for the Linux kernel it will (AFAIK) have no effect.