[Python-checkins] cpython (2.7): Fix comment blocks. Adjust blocksize to a power-of-two for better divmod

Fri Jun 14 07:06:49 CEST 2013

http://hg.python.org/cpython/rev/5accb0ac8bfb
changeset: 84116:5accb0ac8bfb
branch: 2.7
parent: 84095:ca8e86711403
user: Raymond Hettinger <python at rcn.com>
date: Fri Jun 14 01:06:33 2013 -0400
summary:
 Fix comment blocks. Adjust blocksize to a power-of-two for better divmod computations.
files:
 Lib/test/test_deque.py | 2 +-
 Modules/_collectionsmodule.c | 18 +++++++-----------
 2 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/Lib/test/test_deque.py b/Lib/test/test_deque.py
--- a/Lib/test/test_deque.py
+++ b/Lib/test/test_deque.py
@@ -522,7 +522,7 @@
 
 @test_support.cpython_only
 def test_sizeof(self):
- BLOCKLEN = 62
+ BLOCKLEN = 64
 basesize = test_support.calcobjsize('2P4PlP')
 blocksize = struct.calcsize('2P%dP' % BLOCKLEN)
 self.assertEqual(object.__sizeof__(deque()), basesize)
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -8,12 +8,13 @@
 */
 
 /* The block length may be set to any number over 1. Larger numbers
- * reduce the number of calls to the memory allocator but take more
- * memory. Ideally, BLOCKLEN should be set with an eye to the
- * length of a cache line.
+ * reduce the number of calls to the memory allocator, give faster
+ * indexing and rotation, and reduce the link::data overhead ratio.
+ * Ideally, the block length should be a power-of-two for faster
+ * division/modulo computations during indexing.
 */
 
-#define BLOCKLEN 62
+#define BLOCKLEN 64
 #define CENTER ((BLOCKLEN - 1) / 2)
 
 /* A `dequeobject` is composed of a doubly-linked list of `block` nodes.
@@ -58,13 +59,8 @@
 static block *
 newblock(block *leftlink, block *rightlink, Py_ssize_t len) {
 block *b;
- /* To prevent len from overflowing PY_SSIZE_T_MAX on 64-bit machines, we
- * refuse to allocate new blocks if the current len is dangerously
- * close. There is some extra margin to prevent spurious arithmetic
- * overflows at various places. The following check ensures that
- * the blocks allocated to the deque, in the worst case, can only
- * have PY_SSIZE_T_MAX-2 entries in total.
- */
+ /* To prevent len from overflowing PY_SSIZE_T_MAX on 32-bit machines, we
+ * refuse to allocate new blocks if the current len is nearing overflow. */
 if (len >= PY_SSIZE_T_MAX - 2*BLOCKLEN) {
 PyErr_SetString(PyExc_OverflowError,
 "cannot add more blocks to the deque");
-- 
Repository URL: http://hg.python.org/cpython
