update doc

Author: giampaolo

Doc/library/shutil.rst

@@ -412,6 +412,10 @@ On macOS `fcopyfile`_ is used to copy the file content (not metadata).
 On Linux, Solaris and other POSIX platforms where :func:`os.sendfile` supports
 copies between 2 regular file descriptors :func:`os.sendfile` is used.
 
+On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
+instead of 16 KiB) and if file size >= 128 MiB a :func:`memoryview`-based
+variant of :func:`shutil.copyfileobj` is used.
+
 If the fast-copy operation fails and no data was written in the destination
 file then shutil will silently fallback on using less efficient
 :func:`copyfileobj` function internally.

Doc/whatsnew/3.8.rst

@@ -100,8 +100,9 @@ Optimizations
   "fast-copy" means that the copying operation occurs within the kernel,
   avoiding the use of userspace buffers in Python as in
   "``outfd.write(infd.read())``".
-  Also, :func:`shutil.copyfile` default buffer size on Windows was increased
-  from 16 KiB to 1 MiB.
+  On Windows :func:`shutil.copyfile` default buffer size was increased
+  from 16 KiB to 1 MiB and a faster implementation of :func:`shutil.copyfileobj`
+  is used for files bigger than 128 MiB.
   The speedup for copying a 512 MiB file within the same partition is about
   +26% on Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles
   are consumed.

Misc/NEWS.d/next/Library/2018-05-28-23-25-17.bpo-33671.GIdKKi.rst

@@ -2,7 +2,9 @@
 :func:`shutil.copytree` and :func:`shutil.move` use platform-specific
 fast-copy syscalls on Linux, Solaris and macOS in order to copy the file
 more efficiently.
-Also, :func:`shutil.copyfile` default buffer size on Windows was increased
-from 16KiB to 1MiB. The speedup for copying a 512MiB file is about +26% on
-Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles are
-consumed (Contributed by Giampaolo Rodola' in :issue:`25427`.)
+On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
+instead of 16 KiB) and if file size >= 128 MiB a :func:`memoryview`-based
+variant of :func:`shutil.copyfileobj` is used.
+The speedup for copying a 512MiB file is about +26% on Linux, +50% on macOS and
++40% on Windows. Also, much less CPU cycles are consumed.
+(Contributed by Giampaolo Rodola' in :issue:`25427`.)