cpython/Objects/dictnotes.txt

NOTES ON DICTIONARIES
================================

Principal Use Cases for Dictionaries
------------------------------------

Passing keyword arguments
    Typically, one read and one write for 1 to 3 elements.
    Occurs frequently in normal python code.

Class method lookup
    Dictionaries vary in size with 8 to 16 elements being common.
    Usually written once with many lookups.
    When base classes are used, there are many failed lookups
        followed by a lookup in a base class.

Instance attribute lookup and Global variables
    Dictionaries vary in size.  4 to 10 elements are common.
    Both reads and writes are common.

Builtins
    Frequent reads.  Almost never written.
    About 150 interned strings (as of Py3.3).
    A few keys are accessed much more frequently than others.

Uniquification
    Dictionaries of any size.  Bulk of work is in creation.
    Repeated writes to a smaller set of keys.
    Single read of each key.
    Some use cases have two consecutive accesses to the same key.

    * Removing duplicates from a sequence.
        dict.fromkeys(seqn).keys()

    * Counting elements in a sequence.
        for e in seqn:
          d[e] = d.get(e,0) + 1

    * Accumulating references in a dictionary of lists:

        for pagenumber, page in enumerate(pages):
          for word in page:
            d.setdefault(word, []).append(pagenumber)

    Note, the second example is a use case characterized by a get and set
    to the same key.  There are similar use cases with a __contains__
    followed by a get, set, or del to the same key.  Part of the
    justification for d.setdefault is combining the two lookups into one.

Membership Testing
    Dictionaries of any size.  Created once and then rarely changes.
    Single write to each key.
    Many calls to __contains__() or has_key().
    Similar access patterns occur with replacement dictionaries
        such as with the % formatting operator.

Dynamic Mappings
    Characterized by deletions interspersed with adds and replacements.
    Performance benefits greatly from the re-use of dummy entries.

Data Layout
-----------

Dictionaries are composed of 3 components:
The dictobject struct itself
A dict-keys object (keys & hashes)
A values array


Tunable Dictionary Parameters
-----------------------------

* PyDict_STARTSIZE. Starting size of dict (unless an instance dict).
    Currently set to 8. Must be a power of two.
    New dicts have to zero-out every cell.
    Increasing improves the sparseness of small dictionaries but costs
    time to read in the additional cache lines if they are not already
    in cache. That case is common when keyword arguments are passed.
    Prior to version 3.3, PyDict_MINSIZE was used as the starting size
    of a new dict.

* PyDict_MINSIZE. Minimum size of a dict.
    Currently set to 4 (to keep instance dicts small).
    Must be a power of two. Prior to version 3.3, PyDict_MINSIZE was
    set to 8.

* USABLE_FRACTION. Maximum dictionary load in PyDict_SetItem.
    Currently set to 2/3. Increasing this ratio makes dictionaries more
    dense resulting in more collisions.  Decreasing it improves sparseness
    at the expense of spreading entries over more cache lines and at the
    cost of total memory consumed.

* Growth rate upon hitting maximum load.  Currently set to *2.
    Raising this to *4 results in half the number of resizes, less
    effort to resize, better sparseness for some (but not all dict sizes),
    and potentially doubles memory consumption depending on the size of
    the dictionary.  Setting to *4 eliminates every other resize step.

* Maximum sparseness (minimum dictionary load).  What percentage
    of entries can be unused before the dictionary shrinks to
    free up memory and speed up iteration?  (The current CPython
    code does not represent this parameter directly.)

* Shrinkage rate upon exceeding maximum sparseness.  The current
    CPython code never even checks sparseness when deleting a
    key.  When a new key is added, it resizes based on the number
    of active keys, so that the addition may trigger shrinkage
    rather than growth.

Tune-ups should be measured across a broad range of applications and
use cases.  A change to any parameter will help in some situations and
hurt in others.  The key is to find settings that help the most common
cases and do the least damage to the less common cases.  Results will
vary dramatically depending on the exact number of keys, whether the
keys are all strings, whether reads or writes dominate, the exact
hash values of the keys (some sets of values have fewer collisions than
others).  Any one test or benchmark is likely to prove misleading.

While making a dictionary more sparse reduces collisions, it impairs
iteration and key listing.  Those methods loop over every potential
entry.  Doubling the size of dictionary results in twice as many
non-overlapping memory accesses for keys(), items(), values(),
__iter__(), iterkeys(), iteritems(), itervalues(), and update().
Also, every dictionary iterates at least twice, once for the memset()
when it is created and once by dealloc().

Dictionary operations involving only a single key can be O(1) unless
resizing is possible.  By checking for a resize only when the
dictionary can grow (and may *require* resizing), other operations
remain O(1), and the odds of resize thrashing or memory fragmentation
are reduced. In particular, an algorithm that empties a dictionary
by repeatedly invoking .pop will see no resizing, which might
not be necessary at all because the dictionary is eventually
discarded entirely.

The key differences between this implementation and earlier versions are:
    1. The table can be split into two parts, the keys and the values.

    2. There is an additional key-value combination: (key, NULL).
       Unlike (<dummy>, NULL) which represents a deleted value, (key, NULL)
       represented a yet to be inserted value. This combination can only occur
       when the table is split.

    3. No small table embedded in the dict,
       as this would make sharing of key-tables impossible.


These changes have the following consequences.
   1. General dictionaries are slightly larger.

   2. All object dictionaries of a single class can share a single key-table,
      saving about 60% memory for such cases.

Results of Cache Locality Experiments
--------------------------------------

Experiments on an earlier design of dictionary, in which all tables were
combined, showed the following:

  When an entry is retrieved from memory, several adjacent entries are also
  retrieved into a cache line.  Since accessing items in cache is *much*
  cheaper than a cache miss, an enticing idea is to probe the adjacent
  entries as a first step in collision resolution.  Unfortunately, the
  introduction of any regularity into collision searches results in more
  collisions than the current random chaining approach.

  Exploiting cache locality at the expense of additional collisions fails
  to payoff when the entries are already loaded in cache (the expense
  is paid with no compensating benefit).  This occurs in small dictionaries
  where the whole dictionary fits into a pair of cache lines.  It also
  occurs frequently in large dictionaries which have a common access pattern
  where some keys are accessed much more frequently than others.  The
  more popular entries *and* their collision chains tend to remain in cache.

  To exploit cache locality, change the collision resolution section
  in lookdict() and lookdict_string().  Set i^=1 at the top of the
  loop and move the  i = (i << 2) + i + perturb + 1 to an unrolled
  version of the loop.

For split tables, the above will apply to the keys, but the value will
always be in a different cache line from the key.