[New-bugs-announce] [issue10408] Denser dicts and linear probing

Antoine Pitrou report at bugs.python.org
Sat Nov 13 15:48:12 CET 2010

New submission from Antoine Pitrou <pitrou at free.fr>:

This is a patch experiment which does two things:
- make dicts denser by making the resize factor 2 instead of 4 for small dicts
- improve cache locality on collisions by using linear probing

It should be noted that these two changes are not independent. Improving cache locality on collisions makes probing cheaper, and in turn should allow to make small dicts denser.

Linear probing is motivated by the fact that collisions can happen frequently.  The comments in dictobject.c seem a bit mistaken:

“If we *usually* find the key we're looking for on the first try (and, it turns out, we usually do -- the table load factor is kept under 2/3, so the odds are solidly in our favor), then it makes best sense to keep the initial index computation dirt cheap.”

According to http://www.cse.ust.hk/~yike/pods10-hashing.pdf, however, things are not so rosy. The average number of probes for successful lookups, depending on the load factor "alpha", is given by:

>>> c = lambda alpha: 0.5 * (1 + 1/(1-alpha))

while the average number of probes for unsuccessful lookups is:

>>> cp = lambda alpha: 0.5 * (1 + 1/(1-alpha)**2)

(note: this is with a perfectly random hash function; I intuitively assume an imperfect random function gives higher figures)

For a load factor of 2/3, we then get:

>>> c(2/3)
>>> cp(2/3)

Since the current non-linear probing schemes guarantees that each probing will access a different cache line, the cache locality of a lookup becomes very poor.

The problem with linear probing, as noted in the comments, is that it degrades performance quite a lot when the hashing function clusters results. The solution I'm proposing is to apply an *initial* perturbation, by multiplying the hash() with a prime number. Multiplication is very fast on modern CPUs, so this doesn't adversely affect performance.

components: Interpreter Core
files: dictopts.patch
keywords: patch
messages: 121139
nosy: mark.dickinson, pitrou, rhettinger, tim_one
priority: normal
severity: normal
status: open
title: Denser dicts and linear probing
type: performance
versions: Python 3.2
Added file: http://bugs.python.org/file19595/dictopts.patch

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