symbian-qemu-0.9.1-12/python-2.6.1/Objects/dictnotes.txt
author martin.trojer@nokia.com
Fri, 31 Jul 2009 15:01:17 +0100
changeset 1 2fb8b9db1c86
permissions -rw-r--r--
Initial QEMU (symbian-qemu-0.9.1-12) import
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
1
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
     1
NOTES ON OPTIMIZING DICTIONARIES
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
     2
================================
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
     3
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
     4
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
     5
Principal Use Cases for Dictionaries
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
     6
------------------------------------
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
     7
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
     8
Passing keyword arguments
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
     9
    Typically, one read and one write for 1 to 3 elements.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    10
    Occurs frequently in normal python code.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    11
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    12
Class method lookup
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    13
    Dictionaries vary in size with 8 to 16 elements being common.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    14
    Usually written once with many lookups.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    15
    When base classes are used, there are many failed lookups
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    16
        followed by a lookup in a base class.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    17
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    18
Instance attribute lookup and Global variables
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    19
    Dictionaries vary in size.  4 to 10 elements are common.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    20
    Both reads and writes are common.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    21
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    22
Builtins
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    23
    Frequent reads.  Almost never written.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    24
    Size 126 interned strings (as of Py2.3b1).
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    25
    A few keys are accessed much more frequently than others.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    26
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    27
Uniquification
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    28
    Dictionaries of any size.  Bulk of work is in creation.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    29
    Repeated writes to a smaller set of keys.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    30
    Single read of each key.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    31
    Some use cases have two consecutive accesses to the same key.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    32
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    33
    * Removing duplicates from a sequence.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    34
        dict.fromkeys(seqn).keys()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    35
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    36
    * Counting elements in a sequence.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    37
        for e in seqn:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    38
          d[e] = d.get(e,0) + 1
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    39
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    40
    * Accumulating references in a dictionary of lists:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    41
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    42
        for pagenumber, page in enumerate(pages):
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    43
          for word in page:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    44
            d.setdefault(word, []).append(pagenumber)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    45
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    46
    Note, the second example is a use case characterized by a get and set
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    47
    to the same key.  There are similar use cases with a __contains__
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    48
    followed by a get, set, or del to the same key.  Part of the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    49
    justification for d.setdefault is combining the two lookups into one.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    50
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    51
Membership Testing
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    52
    Dictionaries of any size.  Created once and then rarely changes.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    53
    Single write to each key.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    54
    Many calls to __contains__() or has_key().
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    55
    Similar access patterns occur with replacement dictionaries
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    56
        such as with the % formatting operator.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    57
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    58
Dynamic Mappings
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    59
    Characterized by deletions interspersed with adds and replacements.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    60
    Performance benefits greatly from the re-use of dummy entries.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    61
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    62
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    63
Data Layout (assuming a 32-bit box with 64 bytes per cache line)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    64
----------------------------------------------------------------
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    65
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    66
Smalldicts (8 entries) are attached to the dictobject structure
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    67
and the whole group nearly fills two consecutive cache lines.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    68
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    69
Larger dicts use the first half of the dictobject structure (one cache
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    70
line) and a separate, continuous block of entries (at 12 bytes each
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    71
for a total of 5.333 entries per cache line).
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    72
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    73
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    74
Tunable Dictionary Parameters
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    75
-----------------------------
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    76
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    77
* PyDict_MINSIZE.  Currently set to 8.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    78
    Must be a power of two.  New dicts have to zero-out every cell.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    79
    Each additional 8 consumes 1.5 cache lines.  Increasing improves
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    80
    the sparseness of small dictionaries but costs time to read in
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    81
    the additional cache lines if they are not already in cache.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    82
    That case is common when keyword arguments are passed.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    83
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    84
* Maximum dictionary load in PyDict_SetItem.  Currently set to 2/3.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    85
    Increasing this ratio makes dictionaries more dense resulting
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    86
    in more collisions.  Decreasing it improves sparseness at the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    87
    expense of spreading entries over more cache lines and at the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    88
    cost of total memory consumed.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    89
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    90
    The load test occurs in highly time sensitive code.  Efforts
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    91
    to make the test more complex (for example, varying the load
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    92
    for different sizes) have degraded performance.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    93
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    94
* Growth rate upon hitting maximum load.  Currently set to *2.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    95
    Raising this to *4 results in half the number of resizes,
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    96
    less effort to resize, better sparseness for some (but not
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    97
    all dict sizes), and potentially doubles memory consumption
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    98
    depending on the size of the dictionary.  Setting to *4
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
    99
    eliminates every other resize step.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   100
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   101
* Maximum sparseness (minimum dictionary load).  What percentage
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   102
    of entries can be unused before the dictionary shrinks to
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   103
    free up memory and speed up iteration?  (The current CPython
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   104
    code does not represent this parameter directly.)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   105
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   106
* Shrinkage rate upon exceeding maximum sparseness.  The current
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   107
    CPython code never even checks sparseness when deleting a
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   108
    key.  When a new key is added, it resizes based on the number
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   109
    of active keys, so that the addition may trigger shrinkage
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   110
    rather than growth.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   111
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   112
Tune-ups should be measured across a broad range of applications and
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   113
use cases.  A change to any parameter will help in some situations and
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   114
hurt in others.  The key is to find settings that help the most common
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   115
cases and do the least damage to the less common cases.  Results will
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   116
vary dramatically depending on the exact number of keys, whether the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   117
keys are all strings, whether reads or writes dominate, the exact
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   118
hash values of the keys (some sets of values have fewer collisions than
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   119
others).  Any one test or benchmark is likely to prove misleading.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   120
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   121
While making a dictionary more sparse reduces collisions, it impairs
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   122
iteration and key listing.  Those methods loop over every potential
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   123
entry.  Doubling the size of dictionary results in twice as many
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   124
non-overlapping memory accesses for keys(), items(), values(),
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   125
__iter__(), iterkeys(), iteritems(), itervalues(), and update().
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   126
Also, every dictionary iterates at least twice, once for the memset()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   127
when it is created and once by dealloc().
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   128
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   129
Dictionary operations involving only a single key can be O(1) unless 
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   130
resizing is possible.  By checking for a resize only when the 
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   131
dictionary can grow (and may *require* resizing), other operations
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   132
remain O(1), and the odds of resize thrashing or memory fragmentation
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   133
are reduced. In particular, an algorithm that empties a dictionary
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   134
by repeatedly invoking .pop will see no resizing, which might
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   135
not be necessary at all because the dictionary is eventually
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   136
discarded entirely.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   137
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   138
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   139
Results of Cache Locality Experiments
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   140
-------------------------------------
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   141
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   142
When an entry is retrieved from memory, 4.333 adjacent entries are also
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   143
retrieved into a cache line.  Since accessing items in cache is *much*
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   144
cheaper than a cache miss, an enticing idea is to probe the adjacent
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   145
entries as a first step in collision resolution.  Unfortunately, the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   146
introduction of any regularity into collision searches results in more
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   147
collisions than the current random chaining approach.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   148
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   149
Exploiting cache locality at the expense of additional collisions fails
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   150
to payoff when the entries are already loaded in cache (the expense
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   151
is paid with no compensating benefit).  This occurs in small dictionaries
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   152
where the whole dictionary fits into a pair of cache lines.  It also
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   153
occurs frequently in large dictionaries which have a common access pattern
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   154
where some keys are accessed much more frequently than others.  The
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   155
more popular entries *and* their collision chains tend to remain in cache.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   156
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   157
To exploit cache locality, change the collision resolution section
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   158
in lookdict() and lookdict_string().  Set i^=1 at the top of the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   159
loop and move the  i = (i << 2) + i + perturb + 1 to an unrolled
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   160
version of the loop.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   161
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   162
This optimization strategy can be leveraged in several ways:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   163
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   164
* If the dictionary is kept sparse (through the tunable parameters),
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   165
then the occurrence of additional collisions is lessened.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   166
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   167
* If lookdict() and lookdict_string() are specialized for small dicts
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   168
and for largedicts, then the versions for large_dicts can be given
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   169
an alternate search strategy without increasing collisions in small dicts
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   170
which already have the maximum benefit of cache locality.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   171
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   172
* If the use case for a dictionary is known to have a random key
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   173
access pattern (as opposed to a more common pattern with a Zipf's law
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   174
distribution), then there will be more benefit for large dictionaries
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   175
because any given key is no more likely than another to already be
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   176
in cache.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   177
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   178
* In use cases with paired accesses to the same key, the second access
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   179
is always in cache and gets no benefit from efforts to further improve
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   180
cache locality.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   181
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   182
Optimizing the Search of Small Dictionaries
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   183
-------------------------------------------
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   184
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   185
If lookdict() and lookdict_string() are specialized for smaller dictionaries,
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   186
then a custom search approach can be implemented that exploits the small
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   187
search space and cache locality.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   188
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   189
* The simplest example is a linear search of contiguous entries.  This is
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   190
  simple to implement, guaranteed to terminate rapidly, never searches
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   191
  the same entry twice, and precludes the need to check for dummy entries.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   192
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   193
* A more advanced example is a self-organizing search so that the most
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   194
  frequently accessed entries get probed first.  The organization
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   195
  adapts if the access pattern changes over time.  Treaps are ideally
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   196
  suited for self-organization with the most common entries at the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   197
  top of the heap and a rapid binary search pattern.  Most probes and
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   198
  results are all located at the top of the tree allowing them all to
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   199
  be located in one or two cache lines.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   200
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   201
* Also, small dictionaries may be made more dense, perhaps filling all
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   202
  eight cells to take the maximum advantage of two cache lines.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   203
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   204
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   205
Strategy Pattern
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   206
----------------
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   207
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   208
Consider allowing the user to set the tunable parameters or to select a
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   209
particular search method.  Since some dictionary use cases have known
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   210
sizes and access patterns, the user may be able to provide useful hints.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   211
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   212
1) For example, if membership testing or lookups dominate runtime and memory
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   213
   is not at a premium, the user may benefit from setting the maximum load
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   214
   ratio at 5% or 10% instead of the usual 66.7%.  This will sharply
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   215
   curtail the number of collisions but will increase iteration time.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   216
   The builtin namespace is a prime example of a dictionary that can
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   217
   benefit from being highly sparse.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   218
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   219
2) Dictionary creation time can be shortened in cases where the ultimate
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   220
   size of the dictionary is known in advance.  The dictionary can be
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   221
   pre-sized so that no resize operations are required during creation.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   222
   Not only does this save resizes, but the key insertion will go
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   223
   more quickly because the first half of the keys will be inserted into
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   224
   a more sparse environment than before.  The preconditions for this
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   225
   strategy arise whenever a dictionary is created from a key or item
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   226
   sequence and the number of *unique* keys is known.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   227
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   228
3) If the key space is large and the access pattern is known to be random,
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   229
   then search strategies exploiting cache locality can be fruitful.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   230
   The preconditions for this strategy arise in simulations and
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   231
   numerical analysis.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   232
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   233
4) If the keys are fixed and the access pattern strongly favors some of
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   234
   the keys, then the entries can be stored contiguously and accessed
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   235
   with a linear search or treap.  This exploits knowledge of the data,
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   236
   cache locality, and a simplified search routine.  It also eliminates
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   237
   the need to test for dummy entries on each probe.  The preconditions
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   238
   for this strategy arise in symbol tables and in the builtin dictionary.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   239
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   240
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   241
Readonly Dictionaries
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   242
---------------------
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   243
Some dictionary use cases pass through a build stage and then move to a
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   244
more heavily exercised lookup stage with no further changes to the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   245
dictionary.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   246
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   247
An idea that emerged on python-dev is to be able to convert a dictionary
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   248
to a read-only state.  This can help prevent programming errors and also
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   249
provide knowledge that can be exploited for lookup optimization.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   250
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   251
The dictionary can be immediately rebuilt (eliminating dummy entries),
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   252
resized (to an appropriate level of sparseness), and the keys can be
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   253
jostled (to minimize collisions).  The lookdict() routine can then
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   254
eliminate the test for dummy entries (saving about 1/4 of the time
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   255
spent in the collision resolution loop).
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   256
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   257
An additional possibility is to insert links into the empty spaces
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   258
so that dictionary iteration can proceed in len(d) steps instead of
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   259
(mp->mask + 1) steps.  Alternatively, a separate tuple of keys can be
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   260
kept just for iteration.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   261
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   262
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   263
Caching Lookups
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   264
---------------
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   265
The idea is to exploit key access patterns by anticipating future lookups
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   266
based on previous lookups.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   267
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   268
The simplest incarnation is to save the most recently accessed entry.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   269
This gives optimal performance for use cases where every get is followed
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   270
by a set or del to the same key.