* Jens Axboe <[email protected]> wrote:
> > > 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.
>
> Ala
> #define SPLICE_F_MOVE (0x01) /* move pages instead of copying */
> +#define SPLICE_F_MORE (0x02) /* expect more data */
ok, nice - something like this should work. The direction of the flag is
a philosophical question i guess: i believe in Linux we prefer to
default to "buffering enabled", i.e. the default flag should be "expect
more data". So maybe it would be better to pass in PLICE_F_END, to
indicate end-of-data. [it doesnt mean 'permanent end', so all the files
still remain open: this could be something like a HTTP 1.1 pipelined
request.]
furthermore, the internal implementation should also get smarter and do
a flush-socket if it would e.g. block on a pagecache page. [we often
prefer a partial packet in such cases instead of having a half-built
packet hang around.]
btw., that 'data boundary' detail is likely lost with the pipe
intermediary solution: there is no direct connection between 'input
file' and 'output socket', so a 'flush now' event doesnt get propagated
in a natural way. (unless we extend pipes with 'data boundary' markers,
or force their flushing, which looks a whole lot of complexity for such
a simple thing.)
straight pagecache->socket splicing on the other hand preserves 'data
boundary' markers in a natural way for two reasons: 1) the input and
output objects are known to the kernel at once 2) because there is no
internal buffering to begin with.
Ingo
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