bpo-36103: change default buffer size of shutil.copyfileobj() (GH-12115)

It is changed from 16KiB to 64KiB.  The previous default value
is used since 1990.

coreutils chose 128 KiB as minimum buffer size for block device I/O.

But shutil.copyfileobj() can be used for non block devices.
So I choose more conservative value.

As my quick benchmark, performance difference between 64KiB and
128 KiB is up to ~5%.  On the other hand, performance difference
between 32 KiB and 64 KiB can be more than 10% when file is fully
buffered.

This is why 64 KiB is rational value.

Doc/library/shutil.rst

@@ -424,7 +424,7 @@ 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 a :func:`memoryview`-based variant of
+instead of 64 KiB) and 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

Lib/shutil.py

@@ -49,7 +49,7 @@ if os.name == 'posix':
 elif _WINDOWS:
     import nt
 
-COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 16 * 1024
+COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 64 * 1024
 _HAS_SENDFILE = posix and hasattr(os, "sendfile")
 _HAS_FCOPYFILE = posix and hasattr(posix, "_fcopyfile")  # macOS
 

Misc/NEWS.d/next/Library/2019-03-01-16-10-01.bpo-36103.n6VgXL.rst

@@ -0,0 +1,3 @@
+Default buffer size used by ``shutil.copyfileobj()`` is changed from 16 KiB
+to 64 KiB on non-Windows platform to reduce system call overhead. Contributed
+by INADA Naoki.