Re: O_DIRECT performance impact on 2.4.18 (was: Re: [PATCH] 2.5.14

[Linus Torvalds]

On Sat, 11 May 2002, Larry McVoy wrote:
>
> You're only halfway right.  You want to avoid the mmap altogether.

See my details on doing the perfect zero-copy copy thing.

The mmap doesn't actually touch the page tables - it ends up being nothing
but a "placeholder".

So if you do

        addr = mmap( ..  MAP_UNCACHED ..  src .. )
        mwrite(dst, addr, len);

then you can think of the mmap as just a "cookie" or the "hose" between
the source and the destination.

Does it have to be an mmap? No. But the advantage of the mmap is that you
can use the mmap to modify the stream if you want to, quite transparently.
And it gives the whole thing a whole lot more flexibility, in that if you
generate the data yourself, you'd just do the mwrite() - again with zero
copy overhead.

And I personally believe that "generate the data yourself" is actually a
very common case. A pure pipe between two places is not what a computer is
good at, or what a computer should be used for.

		Linus