Re: POSIX_FADV_NOREUSE and O_STREAMING behavior in 2.6.7

[David Rolenc]

Is there any technical reason that we couldn't set a flag in the struct 
file* and put code in the read/write calls to handle shrinking the page 
cache? This is similar to what was done with O_STREAMING in 2.4. It 
seems like it would conform more to the behavior expected from 
POSIX_FADV_NOREUSE, and I wouldn't have to make a bunch of system calls :-)

-Dave


Robert Love wrote:

> On Thu, 2004-08-26 at 11:39 -0600, David Rolenc wrote:
>  
>
> > I am trying to get O_STREAMING (Robert Love patch for 2.4) behavior in 
> > 2.6 and just a glance at fadvise.c suggests that POSIX_FADV_NOREUSE is 
> > not implemented any differently than POSIX_FADV_WILLNEED. Am I missing 
> > something?
> >    
>
> Yes, they are currently the same.  Their difference in theory is subtle:
> NOREUSE says "I will use this once in the future" and WILLNEED says "I
> will use this [some number of times] in the future".
>
> So for both of them, you want to keep the data in the page cache.  But
> for NOREUSE you could do drop-behind of the pages.  But Linux does not
> currently do drop-behind, so we have them both do the same thing.
>
>  
>
> > I want to read data from disk with readahead and drop the 
> > data from the page cache as soon as I am done with it. Do I have to call 
> > fadvise with POSIX_FADV_DONTNEED after every read?
> >    
>
> Yes, you do.  Or every couple of reads.
>
> The sort of applications that need DONTNEED already are setup in a way
> to make that easy.  You either have a streaming loop, like:
>
> 	do {
> 		read
> 		process
> 	} while (repeat);
>
> Or you are readings lots of files (like updatedb).
>
> In the first case, just stick the fadvise call after the processing.  In
> the latter case, call fadvise before closing the file.
>
> You don't _have_ to call fadvise after every reading.  Just whenever you
> want to drop the pages.  Whenever it makes sense.
>
> 	Robert Love
>
>
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