Re: fadvise syscall?

[Richard Gooch]

Jeff Garzik writes:
> Andrew Morton wrote:
> 
> >Jeff Garzik wrote:
> >>So... we have madvise, why not fadvise?  I would love the capability for
> >>applications to provide hints to the OS like madvise, but for file
> >>descriptors...
> >>
> >
> >The one hint which I can think of which would be beneficial would
> >be an equivalent to MADV_SEQUENTIAL.  Something which says "this
> >is a big streaming read/write - don't go and evict other stuff because
> >of it".  O_STREAMING perhaps.  Or working dropbehind heuristics,
> >although I suspect that explicit controls will always do better.
> >
> >For MADV_RANDOM, readahead window scaling should get that right.
> >
> >What else were you thinking of?
> >
> 
> Hints for,
> * sequential read
> * sequential write
> * sequential write, where the application considers the data it's 
> writing to be unlikely to be read again any time soon (hopefully 
> implying to the page cache that these pages have low value as cacheable 
> objects)
> * some sort of streaming hints, implying that the application cares a 
> lot about maintaining some minimum i/o rate.  note I said hint, not 
> requirement.  -not- guaranteed-rate-IO.
> 
> I might even go so far as to advocate identifying common usage
> patterns, and creating hint constants for them, even if we don't
> support them in the kernel immediately (if ever).  Makes the
> interface much more future-proof, at the expense of a few integers
> in a 32-bit numberspace, and a few more bytes in the C compiler's
> symbol table.

Here's one that I'd like (came up recently with these 21600x21600x3
images from NASA:-): MADV_REVERSE_SEQUENTIAL. When converting images
from stupid formats which have the origin in the top-left, to formats
which have the origin in the bottom-left (the way god intended), you
can avoid a massive malloc(3) if you read the input file backwards
(basically through llseek(2) steps).

				Regards,

					Richard....
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