Re: O_DIRECT question

[Andrew Morton <akpm@osdl.org>]

On Fri, 12 Jan 2007 15:35:09 -0700
Erik Andersen <andersen@codepoet.org> wrote:

> On Fri Jan 12, 2007 at 05:09:09PM -0500, Linus Torvalds wrote:
> > I suspect a lot of people actually have other reasons to avoid caches. 
> > 
> > For example, the reason to do O_DIRECT may well not be that you want to 
> > avoid caching per se, but simply because you want to limit page cache 
> > activity. In which case O_DIRECT "works", but it's really the wrong thing 
> > to do. We could export other ways to do what people ACTUALLY want, that 
> > doesn't have the downsides.
> 
> I was rather fond of the old O_STREAMING patch by Robert Love,

That was an akpmpatch whcih I did for the Digeo kernel.  Robert picked it
up to dehackify it and get it into mainline, but we ended up deciding that
posix_fadvise() was the way to go because it's standards-based.

It's a bit more work in the app to use posix_fadvise() well.  But the
results will be better.  The app should also use sync_file_range()
intelligently to control its pagecache use.

The problem with all of these things is that the application needs to be
changed, and people often cannot do that.  If we want a general way of
stopping particular apps from swamping pagecache then it'd really need to
be an externally-imposed thing - probably via additional accounting and a
new rlimit.