bruno-roustant commented on issue #1395: SOLR-14365: CollapsingQParser - Avoiding always allocate int[] and float[] with size equals to number of unique values (WIP) URL: https://github.com/apache/lucene-solr/pull/1395#issuecomment-610481938 > so in total we pay 2^22 + 2^23 bytes Why are sizes before and after rehash added? I take an example of a max number of elements of 1 M known ahead. The array size will be exactly 1M, so memory = 1M * element size B. For a load of 1.5%, the map memory = 2^16 * element size B. (nextPowerOf2(15000/0.75)=2^15) So I get a ratio of array mem / map mem = 15.25 For a load of 15%, the map memory = 2^19 * element size. (nextPowerOf2(150000/0.75)=2^18) So I get a ratio of array mem / map mem = 1.9 For a load of 20%, the map memory = 2^20 * element size. (nextPowerOf2(200000/0.75)=2^19) So I get a ratio of array mem / map mem = 0.95 Above 20% load, the array memory is less than the map memory. So if we want to focus on the memory benefit, we can set a threshold at 20% load. If we want the performance we can set the threshold lower.
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