On 2/10/23 2:27 PM, Jens Axboe wrote: > On 2/10/23 2:14?PM, Andy Lutomirski wrote: >> On Fri, Feb 10, 2023 at 12:50 PM Jens Axboe <axboe@xxxxxxxxx> wrote: >>> >>> On 2/10/23 1:44?PM, Linus Torvalds wrote: >>>> On Fri, Feb 10, 2023 at 12:39 PM Jens Axboe <axboe@xxxxxxxxx> wrote: >>>>> >>>>> Right, I'm referencing doing zerocopy data sends with io_uring, using >>>>> IORING_OP_SEND_ZC. This isn't from a file, it's from a memory location, >>>>> but the important bit here is the split notifications and how you >>>>> could wire up a OP_SENDFILE similarly to what Andy described. >>>> >>>> Sure, I think it's much more reasonable with io_uring than with splice itself. >>>> >>>> So I was mainly just reacting to the "strict-splice" thing where Andy >>>> was talking about tracking the page refcounts. I don't think anything >>>> like that can be done at a splice() level, but higher levels that >>>> actually know about the whole IO might be able to do something like >>>> that. >>>> >>>> Maybe we're just talking past each other. >>> >>> Maybe slightly, as I was not really intending to comment on the strict >>> splice thing. But yeah I agree on splice, it would not be trivial to do >>> there. At least with io_uring we have the communication channel we need. >>> And tracking page refcounts seems iffy and fraught with potential >>> issues. >>> >> >> Hmm. >> >> Are there any real-world use cases for zero-copy splice() that >> actually depend on splicing from a file to a pipe and then later from >> the pipe to a socket (or file or whatever)? Or would everything >> important be covered by a potential new io_uring operation that copies >> from one fd directly to another fd? > > I think it makes sense. As Linus has referenced, the sex appeal of > splice is the fact that it is dealing with pipes, and you can access > these internal buffers through other means. But that is probably largely > just something that is sexy design wise, nothing that _really_ matters > in practice. And the pipes do get in the way, for example I had to add > pipe resizing fcntl helpers to bump the size. If you're doing a plain > sendfile, the pipes just kind of get in the way too imho. > > Another upside (from the io_uring) perspective is that splice isn't very > efficient through io_uring, as it requires offload to io-wq. This could > obviously be solved by some refactoring in terms of non-blocking, but it > hasn't really been that relevant (and nobody has complained about it). A > new sendfile op would nicely get around that too as it could be designed > with async in nature, rather than the classic sync syscall model that > splice follows. Speaking of splice/io_uring, Ming posted this today: https://lore.kernel.org/io-uring/20230210153212.733006-1-ming.lei@xxxxxxxxxx/ -- Jens Axboe
