segment_map: Also segment the bucket array #1
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In segmented mode we only applied the segmenting to the values array but not the bucket array. As a result there the pattern of there still being a deallocation followed by an increased allocation when resizing the hash map continues to exist. Further, in environments where the max allocation size is limited because of fragmentation issues this can lead to problems. To avoid both of these issues this patch makes the bucket array use the same datastructure as the values array, i.e.: a `std::vector` when linear and `segmented_vector` when segmented (or the passed datastructure if specified). This extra indirection does add some overhead in the segmented case. Looking at the quick benchmarks we see: Before: ``` | ns/op | op/s | err% | ins/op | cyc/op | IPC | bra/op | miss% | total | benchmarking |--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:------------- | 8,912,995.09 | 112.20 | 0.1% | 225,712,537.08 | 26,628,198.00 | 8.476 | 25,133,812.23 | 0.1% | 1.15 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector iterate while adding then removing` | 65,440,597.50 | 15.28 | 0.1% | 496,971,523.50 | 195,721,929.00 | 2.539 | 64,749,156.50 | 11.2% | 1.44 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector random insert erase` | 63,254,162.50 | 15.81 | 0.1% | 540,753,642.50 | 188,790,381.00 | 2.864 | 101,168,500.00 | 6.3% | 1.39 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector 50% probability to find` | 9,777,270.50 | 102.28 | 0.2% | 281,149,360.00 | 28,833,467.00 | 9.751 | 25,968,567.75 | 0.1% | 1.19 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector iterate while adding then removing` | 220,368,952.00 | 4.54 | 0.2% |2,707,978,150.00 | 659,198,358.00 | 4.108 | 347,649,399.00 | 3.8% | 2.43 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector random insert erase` | 156,887,435.00 | 6.37 | 0.1% |2,166,844,490.00 | 464,728,290.00 | 4.663 | 266,835,027.00 | 2.5% | 1.73 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector 50% probability to find` ``` After: ``` | ns/op | op/s | err% | ins/op | cyc/op | IPC | bra/op | miss% | total | benchmarking |--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:------------- | 8,921,748.31 | 112.09 | 0.1% | 226,313,644.69 | 26,684,106.00 | 8.481 | 25,174,702.92 | 0.1% | 1.18 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector iterate while adding then removing` | 75,578,500.00 | 13.23 | 0.1% | 597,036,791.50 | 226,059,912.00 | 2.641 | 64,865,689.00 | 11.3% | 1.14 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector random insert erase` | 74,928,542.00 | 13.35 | 0.1% | 677,557,943.00 | 223,726,152.00 | 3.029 | 91,606,575.00 | 7.0% | 1.13 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector 50% probability to find` | 10,079,993.00 | 99.21 | 0.4% | 293,716,069.73 | 29,697,236.40 | 9.890 | 25,980,823.83 | 0.1% | 1.20 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector iterate while adding then removing` | 220,081,085.00 | 4.54 | 0.1% |2,721,992,469.00 | 658,245,042.00 | 4.135 | 345,686,575.00 | 3.8% | 2.42 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector random insert erase` | 158,126,693.00 | 6.32 | 0.1% |2,191,768,626.00 | 468,710,736.00 | 4.676 | 267,938,632.00 | 2.5% | 1.74 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector 50% probability to find` ``` If we think this is not unconditionally acceptable then we could possibly add another template parameter (or make IsSegmented an enum) to decide which parts are supposed to be segmented. Fixes martinus#94
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travisdowns
reviewed
Mar 18, 2024
| std::memcpy(m_buckets, other.m_buckets, sizeof(Bucket) * bucket_count()); | ||
| if constexpr (IsSegmented) { | ||
| for (auto i = 0UL; i < bucket_count(); ++i) { | ||
| at(m_buckets, i) = at(other.m_buckets, i); |
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Perhaps in the future this could still do the memcpy for each segment, right?
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Yes, though we'd need some form of fragment iterator API.
dotnwat
reviewed
Mar 18, 2024
It would be one of the candidates I guess yes. |
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My change from martinus#112
Merge into our fork while we sort out the upstreaming part.