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+<title>Hash Skewed Distribution Memory Use Test</title>
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+</head>
+<body>
+<div id="page">
+<h1>Hash-Based Skewed-Distribution Random-Integer <tt>find</tt>
+ Find Timing Test</h1>
+<h2><a name="description" id="description">Description</a></h2>
+<p>This test inserts a number of values with a markedly
+ non-uniform i.i.d. integer keys into a container, then performs
+ a series of finds using <tt>find</tt> . It measures the average
+ time for <tt>find</tt> as a function of the number of values in
+ the containers. The keys are generated as follows. First, a
+ uniform integer is created; it is then shifted left 8 bits.</p>
+<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>
+ 200 200 2100)</p>
+<h2><a name="purpose" id="purpose">Purpose</a></h2>
+<p>The test checks the effect of different range-hashing
+ functions and trigger policies (see <a href="hash_based_containers.html#hash_policies">Design::Associative
+ Containers::Hash-Based Containers::Hash Policies</a> and
+ <a href="hash_based_containers.html#resize_policies">Design::Associative
+ Containers::Hash-Based Containers::Resize Policies</a>).</p>
+<h2><a name="results" id="results">Results</a></h2>
+<p>Figures <a href="#NHG">NHG</a>, <a href="#NHM">NHM</a>, and
+ <a href="#NHL">NHL</a> show the results for various hash-based
+ 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
+ <a href="assoc_performance_tests.html#local"><u>local</u></a>,
+ respectively.</p>
+<div id="NHG_res_div">
+<div id="NHG_gcc">
+<div id="NHG_hash_zlob_random_int_find_timing_test">
+<div id="NHG_assoc">
+<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>
+<ol>
+<li>
+cc_hash_mask_exp_nea_lc_1div8_1div1_nsth_map-
+<a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
+with <tt>Comb_Hash_Fn</tt> = <a href="direct_mask_range_hashing.html"><tt>direct_mask_range_hashing</tt></a>
+, and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
+ with <tt>Size_Policy</tt> = <a href="hash_exponential_size_policy.html"><tt>hash_exponential_size_policy</tt></a>
+, and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
+ with <i>α<sub>min</sub></i> = <i>1/8</i> and <i>α<sub>max</sub></i> = <i>1/1</i></li>
+<li>
+gp_hash_mod_quadp_prime_nea_lc_1div8_1div2_nsth_map-
+<a href="gp_hash_table.html"><tt>gp_hash_table</tt></a>
+ with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
+, <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
+ with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
+, and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
+ with <i>α<sub>min</sub></i> = <i>1/8</i> and <i>α<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>
+</li>
+<li>
+n_hash_map_ncah-
+<tt>std::tr1::unordered_map</tt> with <tt>cache_hash_code</tt> = <tt><b>false</b></tt></li>
+<li>
+cc_hash_mod_prime_nea_lc_1div8_1div1_nsth_map-
+<a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
+with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
+, and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
+ with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
+, and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
+ with <i>α<sub>min</sub></i> = <i>1/8</i> and <i>α<sub>max</sub></i> = <i>1/1</i></li>
+</ol>
+</div><div style="width: 100%; height: 20px"></div></div>
+</div>
+</div>
+</div>
+</div>
+<div id="NHM_res_div">
+<div id="NHM_msvc">
+<div id="NHM_hash_zlob_random_int_find_timing_test">
+<div id="NHM_assoc">
+<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>
+<ol>
+<li>
+n_hash_map_ncah-
+<tt>stdext::hash_map</tt></li>
+<li>
+cc_hash_mask_exp_nea_lc_1div8_1div1_nsth_map-
+<a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
+with <tt>Comb_Hash_Fn</tt> = <a href="direct_mask_range_hashing.html"><tt>direct_mask_range_hashing</tt></a>
+, and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
+ with <tt>Size_Policy</tt> = <a href="hash_exponential_size_policy.html"><tt>hash_exponential_size_policy</tt></a>
+, and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
+ with <i>α<sub>min</sub></i> = <i>1/8</i> and <i>α<sub>max</sub></i> = <i>1/1</i></li>
+<li>
+gp_hash_mod_quadp_prime_nea_lc_1div8_1div2_nsth_map-
+<a href="gp_hash_table.html"><tt>gp_hash_table</tt></a>
+ with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
+, <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
+ with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
+, and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
+ with <i>α<sub>min</sub></i> = <i>1/8</i> and <i>α<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>
+</li>
+<li>
+cc_hash_mod_prime_nea_lc_1div8_1div1_nsth_map-
+<a href="cc_hash_table.html"><tt>cc_hash_table</tt></a>
+with <tt>Comb_Hash_Fn</tt> = <a href="direct_mod_range_hashing.html"><tt>direct_mod_range_hashing</tt></a>
+, and <tt>Resize_Policy</tt> = <a href="hash_standard_resize_policy.html"><tt>hash_standard_resize_policy</tt></a>
+ with <tt>Size_Policy</tt> = <a href="hash_prime_size_policy.html"><tt>hash_prime_size_policy</tt></a>
+, and <tt>Trigger_Policy</tt> = <a href="hash_load_check_resize_trigger.html"><tt>hash_load_check_resize_trigger</tt></a>
+ with <i>α<sub>min</sub></i> = <i>1/8</i> and <i>α<sub>max</sub></i> = <i>1/1</i></li>
+</ol>
+</div><div style="width: 100%; height: 20px"></div></div>
+</div>
+</div>
+</div>
+</div>
+<div id="NHL_res_div">
+<div id="NHL_local">
+<div id="NHL_hash_zlob_random_int_find_timing_test">
+<div id="NHL_assoc">
+<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>
+</div>
+</div>
+</div>
+</div>
+<h2><a name="observations" id="observations">Observations</a></h2>
+<p>In this setting, the keys' distribution is so skewed that
+ the unerlying hash-table type affects performance marginally.
+ (This is in contrast with <a href="hash_text_find_find_timing_test.html">Hash-Based Text
+ <tt>find</tt> Find Timing Test</a> , <a href="hash_random_int_find_find_timing_test.html">Hash-Based
+ Random-Integer <tt>find</tt> Find Timing Test</a> , <a href="hash_random_int_subscript_find_timing_test.html">Hash-Based
+ Random-Integer Subscript Find Timing Test</a> and <a href="hash_random_int_subscript_insert_timing_test.html">Hash-Based
+ Random-Integer Subscript Insert Timing Test</a> .)</p>
+<p>The range-hashing scheme affects performance dramatically. A
+ mask-based range-hashing scheme effectively maps all values
+ into the same bucket. Access degenerates into a search within
+ an unordered linked-list. In Figures <a href="#NHG">NHG</a> and
+ <a href="#NHM">NHM</a> , it should be noted that
+ <tt>std::tr1::unordered_map</tt> and <tt>stdext::hash_map</tt>
+ are hard-wired currently to mod-based and mask-based schemes,
+ respectively.</p>
+<p>When observing the settings of this test, it is apparent
+ that the keys' distribution is far from natural. One might ask
+ if the test is not contrived to show that, in some cases,
+ mod-based range hashing does better than mask-based range
+ hashing. This is, in fact just the case. We did not encounter a
+ more natural case in which mod-based range hashing is better.
+ In our opnion, real-life key distributions are handled better
+ with an appropriate hash function and a mask-based
+ 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>
+ shows an example of handling this a-priori known skewed
+ distribution with a mask-based range-hashing function). If hash
+ performance is bad, a <i>Χ<sup>2</sup></i> test can be used
+ to check how to transform it into a more uniform
+ distribution.</p>
+<p>For this reason, <tt>pb_ds</tt>'s default range-hashing
+ function is mask-based.</p>
+<p><a href="assoc_performance_tests.html#hash_based_types">Observations::Hash-Based
+ Container Types</a> summarizes some observations on hash-based
+ containers; <a href="assoc_performance_tests.html#hash_based_policies">Observations::Hash-Based
+ Containers' Policies</a> summarizes some observations on
+ hash-based containers' policies.</p>
+</div>
+</body>
+</html>
projects / msp430-gcc.git / blobdiff