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  6. <meta name="generator" content="HTML Tidy for Linux/x86 (vers 12 April 2005), see www.w3.org" />
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  7. <title>Hash Skewed Distribution Memory Use Test</title>
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  11. <div id="page">
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  12. <h1>Hash-Based Skewed-Distribution Random-Integer <tt>find</tt>
    13. 

  13.     Find Timing Test</h1>
    14. 

  14. <h2><a name="description" id="description">Description</a></h2>
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  15. <p>This test inserts a number of values with a markedly
    16. 

  16.     non-uniform i.i.d. integer keys into a container, then performs
    17. 

  17.     a series of finds using <tt>find</tt> . It measures the average
    18. 

  18.     time for <tt>find</tt> as a function of the number of values in
    19. 

  19.     the containers. The keys are generated as follows. First, a
    20. 

  20.     uniform integer is created; it is then shifted left 8 bits.</p>
    21. 

  21. <p>(The test was executed with <a href="http://gcc.gnu.org/viewcvs/*checkout*/trunk/libstdc%2B%2B-v3/testsuite/performance/ext/pb_ds/hash_zlob_random_int_find_timing.cc"><tt>hash_zlob_random_int_find_timing_test</tt></a>
    22. 

  22.     200 200 2100)</p>
    23. 

  23. <h2><a name="purpose" id="purpose">Purpose</a></h2>
    24. 

  24. <p>The test checks the effect of different range-hashing
    25. 

  25.     functions and trigger policies (see <a href="hash_based_containers.html#hash_policies">Design::Associative
    26. 

  26.     Containers::Hash-Based Containers::Hash Policies</a> and
    27. 

  27.     <a href="hash_based_containers.html#resize_policies">Design::Associative
    28. 

  28.     Containers::Hash-Based Containers::Resize Policies</a>).</p>
    29. 

  29. <h2><a name="results" id="results">Results</a></h2>
    30. 

  30. <p>Figures <a href="#NHG">NHG</a>, <a href="#NHM">NHM</a>, and
    31. 

  31.     <a href="#NHL">NHL</a> show the results for various hash-based
    32. 

  32.     associative-containers in <a href="assoc_performance_tests.html#gcc"><u>g++</u></a>, <a href="assoc_performance_tests.html#msvc"><u>MSVC++</u></a>, and
    33. 

  33.     <a href="assoc_performance_tests.html#local"><u>local</u></a>,
    34. 

  34.     respectively.</p>
    35. 

  35. <div id="NHG_res_div">
    36. 

  36. <div id="NHG_gcc">
    37. 

  37. <div id="NHG_hash_zlob_random_int_find_timing_test">
    38. 

  38. <div id="NHG_assoc">
    39. 

  39. <div id="NHG_Skewed-distribution_random_int_find_timing_test_using__tt_find_455tt_"><div style="border-style: dotted; border-width: 1px; border-color: lightgray"><h6 class="c1"><a name="NHG" id="NHG"><img src="hash_zlob_random_int_find_timing_test_gcc.png" alt="no image" /></a></h6>NHG: Skewed-distribution random int find timing test using <tt>find</tt> - <a href="assoc_performance_tests.html#gcc">g++</a><p>In the above figure, the names in the legends have the following meaning:</p>
    40. 

  40. <ol>
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  41. <li>
    42. 

  42. cc_hash_mask_exp_nea_lc_1div8_1div1_nsth_map-
    43. 

  43. <a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
    44. 

  44. with <tt>Comb_Hash_Fn</tt> = <a href="direct_mask_range_hashing.html"><tt>direct_mask_range_hashing</tt></a>
    45. 

  45. , and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
    46. 

  46.  with <tt>Size_Policy</tt> = <a href="hash_exponential_size_policy.html"><tt>hash_exponential_size_policy</tt></a>
    47. 

  47. , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
    48. 

  48.  with <i>&alpha;<sub>min</sub></i> = <i>1/8</i> and <i>&alpha;<sub>max</sub></i> = <i>1/1</i></li>
    49. 

  49. <li>
    50. 

  50. gp_hash_mod_quadp_prime_nea_lc_1div8_1div2_nsth_map-
    51. 

  51. <a href="gp_hash_table.html"><tt>gp_hash_table</tt></a>
    52. 

  52.  with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
    53. 

  53. , <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
    54. 

  54.  with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
    55. 

  55. , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
    56. 

  56.  with <i>&alpha;<sub>min</sub></i> = <i>1/8</i> and <i>&alpha;<sub>max</sub></i> = <i>1/2</i>, and <tt>Probe_Fn</tt> = <a href="quadratic_probe_fn.html"><tt>quadratic_probe_fn</tt></a>
    57. 

  57. </li>
    58. 

  58. <li>
    59. 

  59. n_hash_map_ncah-
    60. 

  60. <tt>std::tr1::unordered_map</tt> with <tt>cache_hash_code</tt> = <tt><b>false</b></tt></li>
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  61. <li>
    62. 

  62. cc_hash_mod_prime_nea_lc_1div8_1div1_nsth_map-
    63. 

  63. <a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
    64. 

  64. with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
    65. 

  65. , and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
    66. 

  66.  with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
    67. 

  67. , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
    68. 

  68.  with <i>&alpha;<sub>min</sub></i> = <i>1/8</i> and <i>&alpha;<sub>max</sub></i> = <i>1/1</i></li>
    69. 

  69. </ol>
    70. 

  70. </div><div style="width: 100%; height: 20px"></div></div>
    71. 

  71. </div>
    72. 

  72. </div>
    73. 

  73. </div>
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  74. </div>
    75. 

  75. <div id="NHM_res_div">
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  76. <div id="NHM_msvc">
    77. 

  77. <div id="NHM_hash_zlob_random_int_find_timing_test">
    78. 

  78. <div id="NHM_assoc">
    79. 

  79. <div id="NHM_Skewed-distribution_random_int_find_timing_test_using__tt_find_455tt_"><div style="border-style: dotted; border-width: 1px; border-color: lightgray"><h6 class="c1"><a name="NHM" id="NHM"><img src="hash_zlob_random_int_find_timing_test_msvc.png" alt="no image" /></a></h6>NHM: Skewed-distribution random int find timing test using <tt>find</tt> - <a href="assoc_performance_tests.html#msvc">msvc++</a><p>In the above figure, the names in the legends have the following meaning:</p>
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  80. <ol>
    81. 

  81. <li>
    82. 

  82. n_hash_map_ncah-
    83. 

  83. <tt>stdext::hash_map</tt></li>
    84. 

  84. <li>
    85. 

  85. cc_hash_mask_exp_nea_lc_1div8_1div1_nsth_map-
    86. 

  86. <a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
    87. 

  87. with <tt>Comb_Hash_Fn</tt> = <a href="direct_mask_range_hashing.html"><tt>direct_mask_range_hashing</tt></a>
    88. 

  88. , and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
    89. 

  89.  with <tt>Size_Policy</tt> = <a href="hash_exponential_size_policy.html"><tt>hash_exponential_size_policy</tt></a>
    90. 

  90. , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
    91. 

  91.  with <i>&alpha;<sub>min</sub></i> = <i>1/8</i> and <i>&alpha;<sub>max</sub></i> = <i>1/1</i></li>
    92. 

  92. <li>
    93. 

  93. gp_hash_mod_quadp_prime_nea_lc_1div8_1div2_nsth_map-
    94. 

  94. <a href="gp_hash_table.html"><tt>gp_hash_table</tt></a>
    95. 

  95.  with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
    96. 

  96. , <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
    97. 

  97.  with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
    98. 

  98. , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
    99. 

  99.  with <i>&alpha;<sub>min</sub></i> = <i>1/8</i> and <i>&alpha;<sub>max</sub></i> = <i>1/2</i>, and <tt>Probe_Fn</tt> = <a href="quadratic_probe_fn.html"><tt>quadratic_probe_fn</tt></a>
    100. 

  100. </li>
    101. 

  101. <li>
    102. 

  102. cc_hash_mod_prime_nea_lc_1div8_1div1_nsth_map-
    103. 

  103. <a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
    104. 

  104. with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
    105. 

  105. , and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
    106. 

  106.  with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
    107. 

  107. , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
    108. 

  108.  with <i>&alpha;<sub>min</sub></i> = <i>1/8</i> and <i>&alpha;<sub>max</sub></i> = <i>1/1</i></li>
    109. 

  109. </ol>
    110. 

  110. </div><div style="width: 100%; height: 20px"></div></div>
    111. 

  111. </div>
    112. 

  112. </div>
    113. 

  113. </div>
    114. 

  114. </div>
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  115. <div id="NHL_res_div">
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  116. <div id="NHL_local">
    117. 

  117. <div id="NHL_hash_zlob_random_int_find_timing_test">
    118. 

  118. <div id="NHL_assoc">
    119. 

  119. <div id="NHL_Skewed-distribution_random_int_find_timing_test_using__tt_find_455tt_"><div style = "border-style: dotted; border-width: 1px; border-color: lightgray"><h6 class="c1"><a name="NHL" id= "NHL"><img src="hash_zlob_random_int_find_timing_test_local.png" alt="no image" /></a></h6>NHL: Skewed-distribution random int find timing test using <tt>find</tt> - <a href = "assoc_performance_tests.html#local">local</a></div><div style = "width: 100%; height: 20px"></div></div>
    120. 

  120. </div>
    121. 

  121. </div>
    122. 

  122. </div>
    123. 

  123. </div>
    124. 

  124. <h2><a name="observations" id="observations">Observations</a></h2>
    125. 

  125. <p>In this setting, the keys' distribution is so skewed that
    126. 

  126.     the unerlying hash-table type affects performance marginally.
    127. 

  127.     (This is in contrast with <a href="hash_text_find_find_timing_test.html">Hash-Based Text
    128. 

  128.     <tt>find</tt> Find Timing Test</a> , <a href="hash_random_int_find_find_timing_test.html">Hash-Based
    129. 

  129.     Random-Integer <tt>find</tt> Find Timing Test</a> , <a href="hash_random_int_subscript_find_timing_test.html">Hash-Based
    130. 

  130.     Random-Integer Subscript Find Timing Test</a> and <a href="hash_random_int_subscript_insert_timing_test.html">Hash-Based
    131. 

  131.     Random-Integer Subscript Insert Timing Test</a> .)</p>
    132. 

  132. <p>The range-hashing scheme affects performance dramatically. A
    133. 

  133.     mask-based range-hashing scheme effectively maps all values
    134. 

  134.     into the same bucket. Access degenerates into a search within
    135. 

  135.     an unordered linked-list. In Figures <a href="#NHG">NHG</a> and
    136. 

  136.     <a href="#NHM">NHM</a> , it should be noted that
    137. 

  137.     <tt>std::tr1::unordered_map</tt> and <tt>stdext::hash_map</tt>
    138. 

  138.     are hard-wired currently to mod-based and mask-based schemes,
    139. 

  139.     respectively.</p>
    140. 

  140. <p>When observing the settings of this test, it is apparent
    141. 

  141.     that the keys' distribution is far from natural. One might ask
    142. 

  142.     if the test is not contrived to show that, in some cases,
    143. 

  143.     mod-based range hashing does better than mask-based range
    144. 

  144.     hashing. This is, in fact just the case. We did not encounter a
    145. 

  145.     more natural case in which mod-based range hashing is better.
    146. 

  146.     In our opnion, real-life key distributions are handled better
    147. 

  147.     with an appropriate hash function and a mask-based
    148. 

  148.     range-hashing function. (<a href="http://gcc.gnu.org/viewcvs/*checkout*/trunk/libstdc%2B%2B-v3/testsuite/ext/pb_ds/example/hash_shift_mask.cc"><tt>shift_mask.cc</tt></a>
    149. 

  149.     shows an example of handling this a-priori known skewed
    150. 

  150.     distribution with a mask-based range-hashing function). If hash
    151. 

  151.     performance is bad, a <i>&Chi;<sup>2</sup></i> test can be used
    152. 

  152.     to check how to transform it into a more uniform
    153. 

  153.     distribution.</p>
    154. 

  154. <p>For this reason, <tt>pb_ds</tt>'s default range-hashing
    155. 

  155.     function is mask-based.</p>
    156. 

  156. <p><a href="assoc_performance_tests.html#hash_based_types">Observations::Hash-Based
    157. 

  157.     Container Types</a> summarizes some observations on hash-based
    158. 

  158.     containers; <a href="assoc_performance_tests.html#hash_based_policies">Observations::Hash-Based
    159. 

  159.     Containers' Policies</a> summarizes some observations on
    160. 

  160.     hash-based containers' policies.</p>
    161. 

  161. </div>
    162. 

  162. </body>
    163. 

  163. </html>
    164. 

  164.