On Sep 4, 2015, at 2:16 PM, Anna Schumaker <[email protected]> wrote:
>
> Copy system calls came up during Plumbers a couple of weeks ago,
> because several filesystems (including NFS and XFS) are currently
> working on copy acceleration implementations. We haven't heard from
> Zach Brown in a while, so I volunteered to push his patches upstream
> so individual filesystems don't need to keep writing their own ioctls.
>
> The first three patches are a simple reposting of Zach's patches
> from several months ago, with one minor error code fix. The remaining
> patches add in a fallback mechanism when filesystems don't provide a
> copy function. This is especially useful when doing a server-side
> copy on NFS (using the new COPY operation in NFS v4.2). This fallback
> can be disabled by passing the flag COPY_REFLINK to the system call.
>
> The last patch is a man page patch documenting this new system call,
> including an example program.
>
> I tested the fallback option by using /dev/urandom to generate files
> of varying sizes and copying them. I compared the time to copy
> against that of `cp` just to see if there is a noticable difference.
> I found that runtimes are roughly the same, but in-kernel copy tends
> to use less of the cpu. Values in the tables below are averages
> across multiple trials.
>
>
> /usr/bin/cp | 512 MB | 1024 MB | 1536 MB | 2048 MB
> -------------|-----------|-----------|-----------|-----------
> user | 0.00s | 0.00s | 0.00s | 0.00s
> system | 0.32s | 0.52s | 1.04s | 1.04s
> cpu | 73% | 69% | 62% | 62%
> total | 0.446 | 0.757 | 1.197 | 1.667
>
>
> VFS copy | 512 MB | 1024 MB | 1536 MB | 2048 MB
> -------------|-----------|-----------|-----------|-----------
> user | 0.00s | 0.00s | 0.00s | 0.00s
> system | 0.33s | 0.49s | 0.76s | 0.99s
> cpu | 77% | 62% | 60% | 59%
> total | 0.422 | 0.777 | 1.267 | 1.655
>
>
> Questions? Comments? Thoughts?
This is a bit of a surprising result, since in my testing in the
past, copy_{to/from}_user() is a major consumer of CPU time (50%
of a CPU core at 1GB/s). What backing filesystem did you test on?
In theory, the VFS copy routines should save at least 50% of the
CPU usage since it only needs to make one copy (src->dest) instead
of two (kernel->user, user->kernel). Ideally it wouldn't make any
data copies at all and just pass page references from the source
to the target.
Cheers, Andreas
>
> Anna
>
>
> Anna Schumaker (5):
> btrfs: Add mountpoint checking during btrfs_copy_file_range
> vfs: Remove copy_file_range mountpoint checks
> vfs: Copy should check len after file open mode
> vfs: Copy should use file_out rather than file_in
> vfs: Fall back on splice if no copy function defined
>
> Zach Brown (3):
> vfs: add copy_file_range syscall and vfs helper
> x86: add sys_copy_file_range to syscall tables
> btrfs: add .copy_file_range file operation
>
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> fs/btrfs/ctree.h | 3 +
> fs/btrfs/file.c | 1 +
> fs/btrfs/ioctl.c | 95 ++++++++++++++----------
> fs/read_write.c | 132 +++++++++++++++++++++++++++++++++
> include/linux/copy.h | 6 ++
> include/linux/fs.h | 3 +
> include/uapi/asm-generic/unistd.h | 4 +-
> include/uapi/linux/Kbuild | 1 +
> include/uapi/linux/copy.h | 6 ++
> kernel/sys_ni.c | 1 +
> 12 files changed, 214 insertions(+), 40 deletions(-)
> create mode 100644 include/linux/copy.h
> create mode 100644 include/uapi/linux/copy.h
>
> --
> 2.5.1
>
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Cheers, Andreas
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