Restore the data block size to 62.

rhettinger · rhettinger · commit 77578204d6ae · 2013-07-28T02:39:49.000-07:00
The former block size traded away good fit within cache lines in
order to gain faster division in deque_item().  However, compilers
are getting smarter and can now replace the slow division operation
with a fast integer multiply and right shift.  Accordingly, it makes
sense to go back to a size that lets blocks neatly fill entire
cache-lines.

GCC-4.8 and CLANG 4.0 both compute "x // 62" with something
roughly equivalent to "x * 9520900167075897609 &gt;&gt; 69".
diff --git a/Lib/test/test_deque.py b/Lib/test/test_deque.py
@@ -536,7 +536,7 @@ class C(object):
 
     @support.cpython_only
     def test_sizeof(self):
-        BLOCKLEN = 64
+        BLOCKLEN = 62
         basesize = support.calcobjsize('2P4nlP')
         blocksize = struct.calcsize('2P%dP' % BLOCKLEN)
         self.assertEqual(object.__sizeof__(deque()), basesize)
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
@@ -10,11 +10,14 @@
 /* The block length may be set to any number over 1.  Larger numbers
  * reduce the number of calls to the memory allocator, give faster
  * indexing and rotation, and reduce the link::data overhead ratio.
- * If the block length is a power-of-two, we also get faster
- * division/modulo computations during indexing.
+ *
+ * Ideally, the block length will be set to two less than some
+ * multiple of the cache-line length (so that the full block
+ * including the leftlink and rightlink will fit neatly into
+ * cache lines).
  */
 
-#define BLOCKLEN 64
+#define BLOCKLEN 62
 #define CENTER ((BLOCKLEN - 1) / 2)
 
 /* A `dequeobject` is composed of a doubly-linked list of `block` nodes.
