On 09/04/2015 06:25 PM, Andreas Dilger wrote:
> On Sep 4, 2015, at 2:16 PM, Anna Schumaker <Anna.Schumaker@xxxxxxxxxx> 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?
I tested using XFS against two KVM guests. Maybe something there is adding the extra cpu cycles?
Anna
>
> 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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