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* Optimize the merge if all hashSets are singleLevel In PR(https://github.com/ClickHouse/ClickHouse/pull/50748), it has added new phase `parallelizeMergePrepare` before merge if all the hashSets are not all singleLevel or not all twoLevel. Then it will convert all the singleLevelSet to twoLevelSet in parallel, which will increase the CPU utilization and QPS. But if all the hashtables are singleLevel, it could also benefit from the `parallelizeMergePrepare` optimization in most cases if the hashtable size are not too small. By tuning the Query `SELECT COUNT(DISTINCT SearchPhase) FROM hits_v1` in different threads, we have got the mild threshold 6,000. Test patch with the Query 'SELECT COUNT(DISTINCT Title) FROM hits_v1' on 2x80 vCPUs server. If the threads are less than 48, the hashSets are all twoLevel or mixed by singleLevel and twoLevel. If the threads are over 56, all the hashSets are singleLevel. And the QPS has got at most 2.35x performance gain. Threads Opt/Base 8 100.0% 16 99.4% 24 110.3% 32 99.9% 40 99.3% 48 99.8% 56 183.0% 64 234.7% 72 233.1% 80 229.9% 88 224.5% 96 229.6% 104 235.1% 112 229.5% 120 229.1% 128 217.8% 136 222.9% 144 217.8% 152 204.3% 160 203.2% Signed-off-by: Jiebin Sun <jiebin.sun@intel.com> * Add the comment and explanation for PR#52973 Signed-off-by: Jiebin Sun <jiebin.sun@intel.com> --------- Signed-off-by: Jiebin Sun <jiebin.sun@intel.com>
7 lines
361 B
XML
7 lines
361 B
XML
<test>
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<query>SELECT COUNT(DISTINCT Title) FROM test.hits SETTINGS max_threads = 24</query>
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<query>SELECT COUNT(DISTINCT Title) FROM test.hits SETTINGS max_threads = 56</query>
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<query>SELECT COUNT(DISTINCT Title) FROM test.hits SETTINGS max_threads = 64</query>
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<query>SELECT COUNT(DISTINCT Referer) FROM test.hits SETTINGS max_threads = 22</query>
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</test>
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