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

Scott Dial scott+python-dev at scottdial.com
Mon Jun 24 03:52:52 CEST 2013

On 6/23/2013 8:16 PM, Raymond Hettinger wrote:
> Yes, this is a micro-optimization.  In working on implementing
> deque slicing for 3.4, I restudied the block access patterns.   
> On an appendleft(), popleft() or extendleft() operation, the left link is
> accessed immediately before or after the leftmost entry in the data block. 
> The old struct arrangement can cause an unnecessary cache miss
> when jumping leftward.  This was something I overlooked when I
> originally wrote the code almost a decade ago.

A decade ago, cache lines were 64 bytes, pointers were 4 bytes, and
allocations were 16 byte aligned, so there could never be a cache miss.
Nowadays, cache lines are still 64 bytes but pointers are 8 bytes, and
we still allocating on 16 byte alignment, so you have a 25% chance of a
cache miss now.

Scott Dial
scott at scottdial.com

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