Re: [PATCH] splice support #2

[Jens Axboe]

On Fri, Mar 31 2006, Ingo Molnar wrote:
> 
> * Linus Torvalds <torvalds@xxxxxxxx> wrote:
> 
> >  - The pipe is the buffer #2: it's what allows you to do _other_ things 
> >    with splice that are simply impossible to do with sendfile. Notably, 
> >    splice allows very naturally the "readv/writev" scatter-gather 
> >    behaviour of _mixing_ streams. If you're a web-server, with splice you 
> >    can do
> > 
> > 	write(pipefd, header, header_len);
> > 	splice(file, pipefd, file_len);
> > 	splice(pipefd, socket, total_len);
> > 
> >    (this is all conceptual pseudo-code, of course), and this very 
> >    naturally has none of the issues that sendfile() has with plugging etc. 
> >    There's never any "send header separately and do extra work to make 
> >    sure it is in the same packet as the start of the data".
> 
> with pipe-based buffering this approach has still the very same problems 
> that sendfile() has with packet boundaries, because it's not enough to 
> have "large enough" buffering (like a pipe has), the pipe also has to be 
> drained, and the networking layer has to know the precise boundary of 
> data.
> 
> the right solution to the packet boundary problem is to pass in a proper 
> "does userspace expect more data right now" flag, or to let userspace 
> 'flush' the socket independently - which is independent of the 
> pipe-in-slice issue. This solution already exists: the MSG_MORE flag.

We can add a SPLICE_F_MORE flag for this, right now splice doesn't set
the MSG_MORE flag for the end of the pipe.

-- 
Jens Axboe

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