Messages in this thread | | | Date | Fri, 31 Mar 2006 14:47:54 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] splice support #2 |
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* Jens Axboe <axboe@suse.de> 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.
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