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Date: Mon, 15 Sep 2014 18:13:39 -0400 From: Milosz Tanski <milosz@...in.com> To: Andreas Dilger <adilger@...ger.ca> Cc: LKML <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@...radead.org>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, linux-aio@...ck.org, Mel Gorman <mgorman@...e.de>, Volker Lendecke <Volker.Lendecke@...net.de>, Tejun Heo <tj@...nel.org>, Jeff Moyer <jmoyer@...hat.com> Subject: Re: [RFC PATCH 0/7] Non-blockling buffered fs read (page cache only) Like you Andreas I would like to see a syscall that let you take vectored positions (along with buffers and lengths). However, that's not the problem I'm trying to solve with this patchset which is non-blocking read for filesystem fds. The vectored position read call(s) deserve another submission for a number of the usual reasons. Best, - Milosz On Mon, Sep 15, 2014 at 5:33 PM, Andreas Dilger <adilger@...ger.ca> wrote: > On Sep 15, 2014, at 2:20 PM, Milosz Tanski <milosz@...in.com> 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@...r.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > > Cheers, Andreas > > > > > -- Milosz Tanski CTO 16 East 34th Street, 15th floor New York, NY 10016 p: 646-253-9055 e: milosz@...in.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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