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

[Eric Blake]

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.
---

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.

 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
-- 
2.36.1