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Summary: A better version of `vector<vector>` for some scenarios. Can also be an ok `vector<string>` if not everything is small vectors Basic usecase: building vector<vector> by pushing one element at a time, no reserve. ``` PushBackVecInts_20_0_15 1.28us 782.19K PushBackTapeInts_20_0_15 317.49ns 3.15M ``` Constructing a vector of strings (20 strings). When everything is SSO, there is no point in tape. However, when we allocate, tape begins to win big. ``` ConstructVecSmallStrings_20_0_15 122.31ns 8.18M ConstructTapeSmallStrings_20_0_15 129.04ns 7.75M ConstructVecLargeStrings_20_16_32 319.24ns 3.13M ConstructTapeLargeStrings_20_16_32 147.52ns 6.78M ``` Worst case scenario - 2 small strings. Overhead of tape makes this slower. We could tackle this but it requires very complicated code and so far, not the use case encountered. ``` ConstructVec2SmallStrings 20.95ns 47.74M ConstructTape2SmallStrings 38.83ns 25.75M ``` Differential Revision: D52260714
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This pull request was exported from Phabricator. Differential Revision: D52260714 |
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This pull request has been merged in b4ff536. |
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Summary: X-link: facebook/folly#2109 A better version of `vector<vector>` for some scenarios. Can also be an ok `vector<string>` if not everything is small vectors ## Basic usecase 0: iteration for data, initially not in cache ``` IterateVecIntsCacheMiss_20_0_15 449.93ns 2.22M IterateTapeIntsCacheMiss_20_0_15 352.49ns 2.84M IterateVecStringsCacheMiss_20_0_15 262.35ns 3.81M IterateTapeStringsCacheMiss_20_0_15 222.20ns 4.50M ``` Tape is more cache friendly, very packed and predictable. ## Basic usecase 1: building vector<vector> by pushing one element at a time, no reserve. ``` PushBackVecInts_20_0_15 1.28us 782.19K PushBackTapeInts_20_0_15 317.49ns 3.15M ``` ## Worst case - constructing from a known range of SSO strings Constructing a vector of strings (20 strings). When everything is SSO, there is no point in tape. However, when we allocate, tape begins to win big. ``` ConstructVecSmallStrings_20_0_15 122.31ns 8.18M ConstructTapeSmallStrings_20_0_15 129.04ns 7.75M ConstructVecLargeStrings_20_16_32 319.24ns 3.13M ConstructTapeLargeStrings_20_16_32 147.52ns 6.78M ``` Worst case scenario - 2 small strings. Overhead of tape makes this slower. We could tackle this but it requires very complicated code and so far, not the use case encountered. ``` ConstructVec2SmallStrings 20.95ns 47.74M ConstructTape2SmallStrings 38.83ns 25.75M ``` Reviewed By: dmm-fb Differential Revision: D52260714 fbshipit-source-id: a90e01bc589646a38f2ca22c1b9589b6cd4c7f85
tekawade
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Jan 6, 2024
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@DenisYaroshevskiy shouldn't this be buf.reserve(len(g)); instead?
resize + emplace_back() will increase the size. While reserve will reserve the space and array won't grow dynamically as often.
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Summary:
A better version of
vector<vector>for some scenarios.Can also be an ok
vector<string>if not everything is small vectorsBasic usecase: building vector by pushing one element at a time, no reserve.
Constructing a vector of strings (20 strings). When everything is SSO, there is no point in tape. However, when we allocate, tape begins to win big.
Worst case scenario - 2 small strings. Overhead of tape makes this slower. We could tackle this but it requires very complicated code and so far, not the use case encountered.
Differential Revision: D52260714