[Libguestfs] [PATCH nbdkit] python: Pass memoryview to pwrite()

[Richard W.M. Jones]

On Sat, Nov 23, 2019 at 05:49:05PM +0200, Nir Soffer wrote:
> Passing a memoryview we avoid unneeded copy of the original buffer. On
> the python side memoryview object can be used for slicing, writing to
> file, or sending to socket.
> 
> This may break plugins assuming that the they get a bytearray, but
> good python code should not care about the type of the buffer, only
> about the behaviour.

We can consider breaking changes for the v2 API, so logically it would
be best if this patch was applied on top of my new API series.

Rich.

> Testing with a plugin writing to /dev/null shows 2.7x speedup. Real
> plugin will probably show much smaller improvement.
> 
> Without patch:
> 
> $ time qemu-img convert -p -f raw -O raw -n /var/tmp/disk.img nbd://localhost/
>     (100.00/100%)
> 
> real	0m1.284s
> user	0m0.091s
> sys	0m0.576s
> 
> With patch:
> 
> $ time qemu-img convert -p -f raw -O raw -n /var/tmp/disk.img nbd://localhost/
>     (100.00/100%)
> 
> real	0m0.477s
> user	0m0.078s
> sys	0m0.653s
> ---
> 
> More info on how I tested this:
> 
> # Creating test image
> $ dd if=/dev/zero bs=1M count=1024 | tr "\0" "U" > /var/tmp/disk.img
> 
> $ cat zero.py
> import builtins
> 
> def open(readonly):
>     return builtins.open("/dev/zero", "r+b")
> 
> def get_size(h):
>     return 1024**3
> 
> def pwrite(h, buf, offset):
>     h.write(buf)
> 
> def pread(h, count, offset):
>     raise NotImplementedError
> 
> def close(h):
>     h.close()
> 
> # Running nbdkit
> $ ./nbdkit -f -v python zero.py
> 
>  plugins/python/python.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/plugins/python/python.c b/plugins/python/python.c
> index 214fffb..2b4361a 100644
> --- a/plugins/python/python.c
> +++ b/plugins/python/python.c
> @@ -496,8 +496,8 @@ py_pwrite (void *handle, const void *buf,
>      PyErr_Clear ();
>  
>      r = PyObject_CallFunction (fn, "ONL", obj,
> -                               PyByteArray_FromStringAndSize (buf, count),
> -                               offset, NULL);
> +            PyMemoryView_FromMemory ((char *)buf, count, PyBUF_READ),
> +            offset, NULL);
>      Py_DECREF (fn);
>      if (check_python_failure ("pwrite") == -1)
>        return -1;
> -- 
> 2.21.0

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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