On Sep 15, 2014, at 2:20 PM, 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). It's too bad that we are introducing yet another new read/write syscall pair that only allow IO into discontiguous memory regions, but do not allow a single call to access discontiguous file regions (i.e. specify a separate file offset for each iov). Adding syscalls similar to preadv/pwritev() that could take a iovec that specified the file offset+length in addition to the memory address would allow efficient scatter-gather IO in a single syscall. While that is less critical for local filesystems with small syscall latency, it is more important for network filesystems, or in the case of NVRAM-backed filesystems. Cheers, Andreas > It's a very common patern today (samba, libuv, etc..) use a large > threadpool to perform buffered IO operations. They submit the work > form another thread that performs network IO and epoll or other threads > that perform CPU work. This leads to increased latency for processing, > esp. in the case of data that's already cached in the page cache. > > With the new interface the applications will now be able to fetch the > data in their network / cpu bound thread(s) and only defer to a > threadpool if it's not there. In our own application (VLDB) we've > observed a decrease in latency for "fast" request by avoiding unnecessary > queuing and having to swap out current tasks in IO bound work threads. > > I have co-developed these changes with Christoph Hellwig, a whole lot > of his fixes went into the first patch in the series (were squashed > with his approval). > > I am going to post the perf report in a reply-to to this RFC. > > Christoph Hellwig (3): > documentation updates > move flags enforcement to vfs_preadv/vfs_pwritev > check for O_NONBLOCK in all read_iter instances > > Milosz Tanski (4): > Prepare for adding a new readv/writev with user flags. > Define new syscalls readv2,preadv2,writev2,pwritev2 > Export new vector IO (with flags) to userland > O_NONBLOCK flag for readv2/preadv2 > > Documentation/filesystems/Locking | 4 +- > Documentation/filesystems/vfs.txt | 4 +- > arch/x86/syscalls/syscall_32.tbl | 4 + > arch/x86/syscalls/syscall_64.tbl | 4 + > drivers/target/target_core_file.c | 6 +- > fs/afs/internal.h | 2 +- > fs/afs/write.c | 4 +- > fs/aio.c | 4 +- > fs/block_dev.c | 9 ++- > fs/btrfs/file.c | 2 +- > fs/ceph/file.c | 10 ++- > fs/cifs/cifsfs.c | 9 ++- > fs/cifs/cifsfs.h | 12 ++- > fs/cifs/file.c | 30 +++++--- > fs/ecryptfs/file.c | 4 +- > fs/ext4/file.c | 4 +- > fs/fuse/file.c | 10 ++- > fs/gfs2/file.c | 5 +- > fs/nfs/file.c | 13 ++-- > fs/nfs/internal.h | 4 +- > fs/nfsd/vfs.c | 4 +- > fs/ocfs2/file.c | 13 +++- > fs/pipe.c | 7 +- > fs/read_write.c | 146 +++++++++++++++++++++++++++++++------ > fs/splice.c | 4 +- > fs/ubifs/file.c | 5 +- > fs/udf/file.c | 5 +- > fs/xfs/xfs_file.c | 12 ++- > include/linux/fs.h | 16 ++-- > include/linux/syscalls.h | 12 +++ > include/uapi/asm-generic/unistd.h | 10 ++- > mm/filemap.c | 34 +++++++-- > mm/shmem.c | 6 +- > 33 files changed, 306 insertions(+), 112 deletions(-) > > -- > 1.7.9.5 > > -- > 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 Cheers, Andreas
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