Messages in this thread | | | Date | Fri, 31 Mar 2006 11:56:30 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH] splice support #2 |
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On Thu, Mar 30 2006, Linus Torvalds wrote: > > > On Thu, 30 Mar 2006, Jens Axboe wrote: > > On Thu, Mar 30 2006, Ingo Molnar wrote: > > > > > > neat stuff. One question: why do we require fdin or fdout to be a pipe? > > > Is there any fundamental problem with implementing what Larry's original > > > paper described too: straight pagecache -> socket transfers? Without a > > > pipe intermediary forced inbetween. It only adds unnecessary overhead. > > > > No, not a fundamental problem. I think I even hid that in some comment > > in there, at least if it's decipharable by someone else than myself... > > Actually, there _is_ a fundamental problem. Two of them, in fact. > > The reason it goes through a pipe is two-fold: > > - the pipe _is_ the buffer. The reason sendfile() sucks is that sendfile > cannot work with <n> different buffer representations. sendfile() only > works with _one_ buffer representation, namely the "page cache of the > file". > > By using the page cache directly, sendfile() doesn't need any extra > buffering, but that's also why sendfile() fundamentally _cannot_ work > with anything else. You cannot do "sendfile" between two sockets to > forward data from one place to another, for example. You cannot do > sendfile from a streaming device. > > The pipe is just the standard in-kernel buffer between two arbitrary > points. Think of it as a scatter-gather list with a wait-queue. That's > what a pipe _is_. Trying to get rid of the pipe totally misses the > whole point of splice(). > > Now, we could have a splice call that has an _implicit_ pipe, ie if > neither side is a pipe, we could create a temporary pipe and thus > allow what looks like a direct splice. But the pipe should still be > there. > > - 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". > > So having a separate buffer even when you _do_ have a buffer like the > page cache is still something you want to do. > > So there.
My point was mainly that the buffer itself need not necessarily be a pipe, it could be implemented with a pipe just using the same buffer type. But I guess it doesn't make much sense, the pipe has nice advantages in itself.
-- Jens Axboe
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