This change mirrors what we did for ConcurrentHashMap in https://github.com/openjdk/jdk/pull/17116. When we add all entries from one map to anther, we should resize that map to the size of the sum of both maps.
I used the command below to run the benchmarks. I set a high heap to reduce garbage collection noise. java -Xms25G -jar benchmarks.jar -p size=100000 -p addSize=100000 -gc true org.openjdk.bench.java.util.HashMapBench Before change Benchmark (addSize) (mapType) (size) Mode Cnt Score Error Units HashMapBench.putAll 100000 HASH_MAP 100000 avgt 4 22.927 ± 3.170 ms/op HashMapBench.putAll 100000 LINKED_HASH_MAP 100000 avgt 4 25.198 ± 2.189 ms/op After change Benchmark (addSize) (mapType) (size) Mode Cnt Score Error Units HashMapBench.putAll 100000 HASH_MAP 100000 avgt 4 16.780 ± 0.526 ms/op HashMapBench.putAll 100000 LINKED_HASH_MAP 100000 avgt 4 19.721 ± 0.349 ms/op We see about average time improvements of 26% in HashMap and 20% in LinkedHashMap. ------------- Commit messages: - 8324573: HashMap::putAll should resize to sum of both map sizes Changes: https://git.openjdk.org/jdk/pull/17544/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17544&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8324573 Stats: 39 lines in 2 files changed: 13 ins; 7 del; 19 mod Patch: https://git.openjdk.org/jdk/pull/17544.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/17544/head:pull/17544 PR: https://git.openjdk.org/jdk/pull/17544
