Re: zero-copy read() interface

["Jörn Engel" <joern@wohnheim.fh-wedel.de>]

On Thu, 18 August 2005 12:41:32 +0200, Folkert van Heusden wrote:
>
> > Just use mmap().  Unlike your proposal, it cooperates with the page
> > cache.
> 
> Doesn't that one also use copying? I've also heard that using mmap is
> expensive due to pagefaulting. I've found, for example, that copying a
> 1.3GB file using read/write instead of mmap & memcpy is seconds faster.

Since you don't consider device DMA to be a copy, no, it doesn't.  The
data is transferred into page cache, then the pages are mapped into
your processes memory without an additional copy.

The pagefaulting isn't free either, I agree.  And for streaming
accesses like your copy of a large file, taking one fault per 4k
copied is not an ideal case.

Most likely you'd want Linus' pipe stuff for copying a large file
without ever looking at the data.  That work is still unfinished,
though, and I'm currently lacking time to work on it.

Jörn

-- 
He that composes himself is wiser than he that composes a book.
-- B. Franklin