Messages in this thread | | | Date | Wed, 2 Oct 2013 06:31:36 -0700 (PDT) | From | David Lang <> | Subject | Re: [RFC] extending splice for copy offloading |
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On Wed, 2 Oct 2013, Jan Kara wrote:
> On Tue 01-10-13 12:58:17, Zach Brown wrote: >>> - app calls splice(from, 0, to, 0, SIZE_MAX) >>> 1) VFS calls ->direct_splice(from, 0, to, 0, SIZE_MAX) >>> 1.a) fs reflinks the whole file in a jiffy and returns the size of the file >>> 1 b) fs does copy offload of, say, 64MB and returns 64M >>> 2) VFS does page copy of, say, 1MB and returns 1MB >>> - app calls splice(from, X, to, X, SIZE_MAX) where X is the new offset >> >> (It's not SIZE_MAX. It's MAX_RW_COUNT. INT_MAX with some >> PAGE_CACHE_SIZE rounding noise. For fear of weird corners of fs code >> paths that still use int, one assumes.) >> >>> The point is: the app is always doing the same (incrementing offset >>> with the return value from splice) and the kernel can decide what is >>> the best size it can service within a single uninterruptible syscall. >>> >>> Wouldn't that work? >> >> It seems like it should, if people are willing to allow splice() to >> return partial counts. Quite a lot of IO syscalls technically do return >> partial counts today if you try to write > MAX_RW_COUNT :). > Yes. Also POSIX says that application must handle such case for read & > write. But in practice programmers are lazy. > >> But returning partial counts on the order of a handful of megs that the >> file systems make up as the point of diminishing returns is another >> thing entirely. I can imagine people being anxious about that. >> >> I guess we'll find out! > Return 4 KB once in a while to screw up buggy applications from the > start :-p
or at least have a debugging option early on that does this so people can use it to find such buggy apps.
David Lang
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