Re: [PATCH] O_STREAMING - flag for optimal streaming I/O

From: Erik Andersen (
Date: Thu Oct 10 2002 - 04:10:00 EST

On Thu Oct 10, 2002 at 10:33:36AM +0200, Giuliano Pochini wrote:
> On 10-Oct-2002 Erik Andersen wrote:
> > I don't think grep is a very good candidate for O_STREAMING. I
> > usually want the stuff I grep to stay in cache. O_STREAMING is
> > much better suited to applications like ogle, vlc, xine, xmovie,
> > xmms etc since there is little reason for the OS to cache things
> > like songs and movies you aren't likely to hear/see again any
> > time soon.
> The kernel already have cache pruning algorithm. O_STREAMING logic
> should not clear caches if there is no need to do that. We could

The entire point of O_STREAMING is to let user space specify
policy. If user space user space knows with 100% certainty that
the data being read/written from a particular file descriptor is
use-once-and-discard data, then it makes sense to honor that
hint. In this case, user space knows best and can set policy on
a per file descriptor basis.

Note that most applications do not want to use this flag. But
for a few applications it just just perfect. For example, if I
am playing a DVD there is absolutely no point in the kernel
trying to cache the content of the DVD. A DVD has way too much
content for caching it to do any good, and since most people
watch a DVD once through from beginning to end, there is no point
stuffing the DVD's content into the pagecache, thereby crowding
out other things that should remain in cache.


Erik B. Andersen   
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