Re: [PATCH v1 0/8] VFS: In-kernel copy system call

[Anna Schumaker <Anna.Schumaker@netapp.com>]

On 09/04/2015 06:25 PM, Andreas Dilger wrote:
> On Sep 4, 2015, at 2:16 PM, Anna Schumaker <Anna.Schumaker@netapp.com> 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
> 
> 
> 
> 
>