On Fri, Sep 19, 2014 at 10:42 AM, Jonathan Corbet <corbet@xxxxxxx> wrote: > On Wed, 17 Sep 2014 22:20:45 +0000 > Milosz Tanski <milosz@xxxxxxxxx> wrote: > >> This patcheset introduces an ability to perform a non-blocking read from >> regular files in buffered IO mode. This works by only for those filesystems >> that have data in the page cache. >> >> It does this by introducing new syscalls new syscalls readv2/writev2 and >> preadv2/pwritev2. These new syscalls behave like the network sendmsg, recvmsg >> syscalls that accept an extra flag argument (O_NONBLOCK). > > So I'm trying to understand the reasoning behind this approach so I can > explain it to others. When you decided to add these syscalls, you > ruled out some other approaches that have been out there for a while. > I assume that, before these syscalls can be merged, people will want to > understand why you did that. So I'll ask the dumb questions: > > - Non-blocking I/O has long been supported with a well-understood set > of operations - O_NONBLOCK and fcntl(). Why do we need a different > mechanism here - one that's only understood in the context of > buffered file I/O? I assume you didn't want to implement support > for poll() and all that, but is that a good enough reason to add a > new Linux-specific non-blocking I/O technique? I realized that I didn't answer this question well in my other long email. O_NONBLOCK doesn't work on files under any commonly used OS, and people have gotten use to this behavior so I doubt we could change that without breaking a lot of folks applications. If you want to ignore my other long email, what I realized that I could solve a lot of problems if I had a syscall like recvmsg that takes a MSG_NONBLOCK argument that worked on regular files (not sockets) and thus readv2/preadv2 was born. > > - Patches adding fincore() have been around since at least 2010; see, > for example, https://lwn.net/Articles/371538/ or > https://lwn.net/Articles/604640/. It seems this could be used in > favor of four new read() syscalls; is there a reason it's not > suitable for your use case? > > - Patches adding buffered support for AIO have been around since at > least 2003 - https://lwn.net/Articles/24422/, for example. I guess > I don't really have to ask why you don't want to take that > approach! :) > > Apologies for my ignorance here; that's what I get for hanging around > with the MM folks at LSFMM, I guess. Anyway, I suspect I'm not the > only one who would appreciate any background you could give here. > > Thanks, > > jon -- Milosz Tanski CTO 16 East 34th Street, 15th floor New York, NY 10016 p: 646-253-9055 e: milosz@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
