RFR: 8324573: HashMap::putAll should resize to sum of both map sizes

[Chen Liang]

On Wed, 24 Jan 2024 00:26:09 GMT, Joshua Cao <duke at openjdk.org> wrote:

> 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.

Then we might need some statistics on how often `putAll` replaces existing mappings, ranging from none at all to completely. For example, `Collectors.toMap` would never replace existing mappings, even though it doesn't use `putAll` (it can probably call putAll and throw exception if the new size isn't 2 old sizes added up)

The current allocation assumes putting replaces all existing mappings, which I don't think is quite applicable.

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PR Comment: https://git.openjdk.org/jdk/pull/17544#issuecomment-1908819980