[Libguestfs] [libnbd PATCH] python: Speed up pread

[Richard W.M. Jones]

On Fri, May 27, 2022 at 04:44:16PM -0500, Eric Blake wrote:
> Instead of malloc'ing a C buffer, nbd_pread()ing into it, then copying
> it into an immutable Python bytes object, we can instead pre-create a
> correctly-sized Python bytearray object, then nbd_pread() directly
> into that object's underlying bytes.
> 
> While the data copying might not be the bottleneck compared to the
> networking costs, it does have noticeable results; on my machine:
> 
> $ export script='
> m=1024*1024
> size=h.get_size()
> for i in range(size // m):
>   buf = h.pread(m, m*i)
> '
> $ time ./run nbdkit -U - pattern 10G --run 'nbdsh -u $uri -c "$script"'
> 
> sped up from about 7.8s pre-patch to about 7.1s post-patch,
> approximately a 10% speedup.
> 
> Note that this slightly changes the python API: h.pread[_structured]
> now returns a mutable 'bytearray' object, rather than an immutable
> 'bytes' object.  This makes it possible to modify the just-read string
> in place, rather than having to create yet another memory buffer for
> any modifications, which offers even more speedups when writing a
> read-modify-write paradigm in python.  But the change is
> backwards-compatible - python already states that a read-write buffer
> may be returned instead of readonly for any client that already
> operated only on a buffer in a readonly manner.

Although this is not an API change, in general we have no obligation
to maintain backwards compat for the Python API.  (The C API is quite
a different matter of course.)  Nevertheless, it's nice not to break
things.

> Note that h.pread is more like Python read() semantics in creating a
> buffer, while h.aio_pread is more like Python readinto() semantics in
> modifying a passed-in buffer.  But now that both code paths have a
> python object prior to calling into the C API, my next task is to
> improve the h.*pread_structured callback function to pass its buffer
> as a slice of the Python input buffer, rather than doing yet another
> round of pointless memcpy from C into python objects.

You might want to have a look at the nbdkit Python bindings for
inspiration because Nir made those either zero- or low-copy (I forget
exactly which).

>  generator/Python.ml | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/generator/Python.ml b/generator/Python.ml
> index 4ab18f6..1c4446e 100644
> --- a/generator/Python.ml
> +++ b/generator/Python.ml
> @@ -1,6 +1,6 @@
>  (* hey emacs, this is OCaml code: -*- tuareg -*- *)
>  (* nbd client library in userspace: Python bindings
> - * Copyright (C) 2013-2021 Red Hat Inc.
> + * Copyright (C) 2013-2022 Red Hat Inc.
>   *
>   * This library is free software; you can redistribute it and/or
>   * modify it under the terms of the GNU Lesser General Public
> @@ -293,7 +293,7 @@ let
>      | BytesIn (n, _) ->
>         pr "  Py_buffer %s = { .obj = NULL };\n" n
>      | BytesOut (n, count) ->
> -       pr "  char *%s = NULL;\n" n;
> +       pr "  PyObject *%s = NULL;\n" n;
>         pr "  Py_ssize_t %s;\n" count
>      | BytesPersistIn (n, _)
>      | BytesPersistOut (n, _) ->
> @@ -432,8 +432,8 @@ let
>      | Bool _ -> ()
>      | BytesIn _ -> ()
>      | BytesOut (n, count) ->
> -       pr "  %s = malloc (%s);\n" n count;
> -       pr "  if (%s == NULL) { PyErr_NoMemory (); goto out; }\n" n
> +       pr "  %s = PyByteArray_FromStringAndSize (NULL, %s);\n" n count;
> +       pr "  if (%s == NULL) goto out;\n" n
>      | BytesPersistIn (n, _) | BytesPersistOut (n, _) ->
>         pr "  %s_buf = nbd_internal_py_get_aio_buffer (%s);\n" n n;
>         pr "  if (!%s_buf) goto out;\n" n;
> @@ -479,7 +479,7 @@ let
>      function
>      | Bool n -> pr ", %s" n
>      | BytesIn (n, _) -> pr ", %s.buf, %s.len" n n
> -    | BytesOut (n, count) -> pr ", %s, %s" n count
> +    | BytesOut (n, count) -> pr ", PyByteArray_AS_STRING (%s), %s" n count
>      | BytesPersistIn (n, _)
>      | BytesPersistOut (n, _) -> pr ", %s_buf->data, %s_buf->len" n n
>      | Closure { cbname } -> pr ", %s" cbname
> @@ -524,8 +524,9 @@ let
>    let use_ret = ref true in
>    List.iter (
>      function
> -    | BytesOut (n, count) ->
> -       pr "  py_ret = PyBytes_FromStringAndSize (%s, %s);\n" n count;
> +    | BytesOut (n, _) ->
> +       pr "  py_ret = %s;\n" n;
> +       pr "  %s = NULL;\n" n;
>         use_ret := false
>      | Bool _
>      | BytesIn _
> @@ -572,7 +573,7 @@ let
>      | BytesIn (n, _) ->
>         pr "  if (%s.obj)\n" n;
>         pr "    PyBuffer_Release (&%s);\n" n
> -    | BytesOut (n, _) -> pr "  free (%s);\n" n
> +    | BytesOut (n, _) -> pr "  Py_XDECREF (%s);\n" n
>      | BytesPersistIn _ | BytesPersistOut _ -> ()
>      | Closure { cbname } ->
>         pr "  free_user_data (%s_user_data);\n" cbname

Looks good,

Reviewed-by: Richard W.M. Jones <rjones at redhat.com>

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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