On Fri, Mar 31 2006, Ingo Molnar wrote:
>
> * 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.]
Hmm I do prefer a MORE as to an END, as it will always give predictable
results. The MORE is a performance hit, where as a missing END wont
necessarily be detected until you start looking at latencies.
> 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.]
That's a really good idea! Wont be too much trouble to add, probably the
easiest is to add a 'wait' variable to the ->map() function or something
along those lines.
> 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.)
Probably not.
--
Jens Axboe
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