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

[Erik Andersen <andersen@codepoet.org>]

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

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Erik B. Andersen             http://codepoet-consulting.com/
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