On Jul 29, 10:24pm, Andi Kleen wrote:
>
> >
> > In 2.4, does the 'read actor' (ie, for ext2) optimize the case where
> > the part of the I/O request being handled has a user-level address that
> > is page aligned, and the requested bytes to transfer are at least one
> > full page? Ie, does it handle the 'copy' by simply remapping the I/O
> > page directly into the user's address space (avoiding a copy)?
>
> No, because you cannot avoid the copy anyways because you need to maintain
> cache coherency in the page cache. If you want zero copy IO use mmap().
Couldn't that be taken care of, by implementing copy-on-write semantics
on the user-level page that has been (hypothetically) remapped by
the "copy agent"? It would seem that the common case would be not to
modify the data buffer that was just read, and the copy-on-write would
be typically unneeded.
Does glibc implement fread() and friends using mmap(), when reading
from a file? Would there be something to be gained from that?
Alternatively, if the file system implemented zero copy semantics
in read(), there'd be no need to push the optimization to a higer
level (ie, into glibc).
