libstdc++-v3/docs/html/ext/pb_ds/hash_zlob_random_int_find_find_timing_test.html

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   7 <title>Hash Skewed Distribution Memory Use Test</title>
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  12 <h1>Hash-Based Skewed-Distribution Random-Integer <tt>find</tt>
  13     Find Timing Test</h1>
  14 <h2><a name="description" id="description">Description</a></h2>
  15 <p>This test inserts a number of values with a markedly
  16     non-uniform i.i.d. integer keys into a container, then performs
  17     a series of finds using <tt>find</tt> . It measures the average
  18     time for <tt>find</tt> as a function of the number of values in
  19     the containers. The keys are generated as follows. First, a
  20     uniform integer is created; it is then shifted left 8 bits.</p>
  21 <p>(The test was executed with <a href="../../../../testsuite/performance/ext/pb_ds/hash_zlob_random_int_find_timing.cc"><tt>hash_zlob_random_int_find_timing_test</tt></a>
  22     200 200 2100)</p>
  23 <h2><a name="purpose" id="purpose">Purpose</a></h2>
  24 <p>The test checks the effect of different range-hashing
  25     functions and trigger policies (see <a href="hash_based_containers.html#hash_policies">Design::Associative
  26     Containers::Hash-Based Containers::Hash Policies</a> and
  27     <a href="hash_based_containers.html#resize_policies">Design::Associative
  28     Containers::Hash-Based Containers::Resize Policies</a>).</p>
  29 <h2><a name="results" id="results">Results</a></h2>
  30 <p>Figures <a href="#NHG">NHG</a>, <a href="#NHM">NHM</a>, and
  31     <a href="#NHL">NHL</a> show the results for various hash-based
  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     <a href="assoc_performance_tests.html#local"><u>local</u></a>,
  34     respectively.</p>
  35 <div id="NHG_res_div">
  36 <div id="NHG_gcc">
  37 <div id="NHG_hash_zlob_random_int_find_timing_test">
  38 <div id="NHG_assoc">
  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 <ol>
  41 <li>
  42 cc_hash_mask_exp_nea_lc_1div8_1div1_nsth_map-
  43 <a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
  44 with <tt>Comb_Hash_Fn</tt> = <a href="direct_mask_range_hashing.html"><tt>direct_mask_range_hashing</tt></a>
  45 , and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
  46  with <tt>Size_Policy</tt> = <a href="hash_exponential_size_policy.html"><tt>hash_exponential_size_policy</tt></a>
  47 , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
  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 <li>
  50 gp_hash_mod_quadp_prime_nea_lc_1div8_1div2_nsth_map-
  51 <a href="gp_hash_table.html"><tt>gp_hash_table</tt></a>
  52  with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
  53 , <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
  54  with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
  55 , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
  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 </li>
  58 <li>
  59 n_hash_map_ncah-
  60 <tt>std::tr1::unordered_map</tt> with <tt>cache_hash_code</tt> = <tt><b>false</b></tt></li>
  61 <li>
  62 cc_hash_mod_prime_nea_lc_1div8_1div1_nsth_map-
  63 <a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
  64 with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
  65 , and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
  66  with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
  67 , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
  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 </ol>
  70 </div><div style="width: 100%; height: 20px"></div></div>
  71 </div>
  72 </div>
  73 </div>
  74 </div>
  75 <div id="NHM_res_div">
  76 <div id="NHM_msvc">
  77 <div id="NHM_hash_zlob_random_int_find_timing_test">
  78 <div id="NHM_assoc">
  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>
  80 <ol>
  81 <li>
  82 n_hash_map_ncah-
  83 <tt>stdext::hash_map</tt></li>
  84 <li>
  85 cc_hash_mask_exp_nea_lc_1div8_1div1_nsth_map-
  86 <a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
  87 with <tt>Comb_Hash_Fn</tt> = <a href="direct_mask_range_hashing.html"><tt>direct_mask_range_hashing</tt></a>
  88 , and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
  89  with <tt>Size_Policy</tt> = <a href="hash_exponential_size_policy.html"><tt>hash_exponential_size_policy</tt></a>
  90 , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
  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 <li>
  93 gp_hash_mod_quadp_prime_nea_lc_1div8_1div2_nsth_map-
  94 <a href="gp_hash_table.html"><tt>gp_hash_table</tt></a>
  95  with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
  96 , <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
  97  with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
  98 , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
  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 </li>
 101 <li>
 102 cc_hash_mod_prime_nea_lc_1div8_1div1_nsth_map-
 103 <a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
 104 with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
 105 , and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
 106  with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
 107 , and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
 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 </ol>
 110 </div><div style="width: 100%; height: 20px"></div></div>
 111 </div>
 112 </div>
 113 </div>
 114 </div>
 115 <div id="NHL_res_div">
 116 <div id="NHL_local">
 117 <div id="NHL_hash_zlob_random_int_find_timing_test">
 118 <div id="NHL_assoc">
 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 </div>
 121 </div>
 122 </div>
 123 </div>
 124 <h2><a name="observations" id="observations">Observations</a></h2>
 125 <p>In this setting, the keys' distribution is so skewed that
 126     the unerlying hash-table type affects performance marginally.
 127     (This is in contrast with <a href="hash_text_find_find_timing_test.html">Hash-Based Text
 128     <tt>find</tt> Find Timing Test</a> , <a href="hash_random_int_find_find_timing_test.html">Hash-Based
 129     Random-Integer <tt>find</tt> Find Timing Test</a> , <a href="hash_random_int_subscript_find_timing_test.html">Hash-Based
 130     Random-Integer Subscript Find Timing Test</a> and <a href="hash_random_int_subscript_insert_timing_test.html">Hash-Based
 131     Random-Integer Subscript Insert Timing Test</a> .)</p>
 132 <p>The range-hashing scheme affects performance dramatically. A
 133     mask-based range-hashing scheme effectively maps all values
 134     into the same bucket. Access degenerates into a search within
 135     an unordered linked-list. In Figures <a href="#NHG">NHG</a> and
 136     <a href="#NHM">NHM</a> , it should be noted that
 137     <tt>std::tr1::unordered_map</tt> and <tt>stdext::hash_map</tt>
 138     are hard-wired currently to mod-based and mask-based schemes,
 139     respectively.</p>
 140 <p>When observing the settings of this test, it is apparent
 141     that the keys' distribution is far from natural. One might ask
 142     if the test is not contrived to show that, in some cases,
 143     mod-based range hashing does better than mask-based range
 144     hashing. This is, in fact just the case. We did not encounter a
 145     more natural case in which mod-based range hashing is better.
 146     In our opnion, real-life key distributions are handled better
 147     with an appropriate hash function and a mask-based
 148     range-hashing function. (<a href="../../../../testsuite/ext/pb_ds/example/hash_shift_mask.cc"><tt>shift_mask.cc</tt></a>
 149     shows an example of handling this a-priori known skewed
 150     distribution with a mask-based range-hashing function). If hash
 151     performance is bad, a <i>&Chi;<sup>2</sup></i> test can be used
 152     to check how to transform it into a more uniform
 153     distribution.</p>
 154 <p>For this reason, <tt>pb_ds</tt>'s default range-hashing
 155     function is mask-based.</p>
 156 <p><a href="assoc_performance_tests.html#hash_based_types">Observations::Hash-Based
 157     Container Types</a> summarizes some observations on hash-based
 158     containers; <a href="assoc_performance_tests.html#hash_based_policies">Observations::Hash-Based
 159     Containers' Policies</a> summarizes some observations on
 160     hash-based containers' policies.</p>
 161 </div>
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