Integrated: 8322149: ConcurrentHashMap smarter presizing for copy constructor and putAll

[Joshua Cao]

On Fri, 15 Dec 2023 01:16:55 GMT, Joshua Cao <duke at openjdk.org> wrote:

> ConcurrentHashMap's copy constructor calls `putAll()` -> `tryPresize()` -> `transfer()`. When coming from the copy constructor, the Map is empty, so there is nothing to transfer. But `transfer()` will still copy all the empty nodes from the old table to the new table.
> 
> This patch avoids this work by only calling `tryPresize()` if the table is already initialized. If `table` is null, the initialization is deferred to `putVal()`, which calls `initTable()`.
> 
> ---
> 
> ### JMH results for testCopyConstructor
> 
> before patch:
> 
> 
> Result "org.openjdk.bench.java.util.concurrent.Maps.testCopyConstructor":
>   937395.686 ±(99.9%) 99074.324 ns/op [Average]
>   (min, avg, max) = (825732.550, 937395.686, 1072024.041), stdev = 92674.184
>   CI (99.9%): [838321.362, 1036470.010] (assumes normal distribution)
> 
> 
> after patch:
> 
> 
> Result "org.openjdk.bench.java.util.concurrent.Maps.testCopyConstructor":
>   620871.469 ±(99.9%) 59195.406 ns/op [Average]
>   (min, avg, max) = (545304.633, 620871.469, 689013.573), stdev = 55371.419
>   CI (99.9%): [561676.063, 680066.875] (assumes normal distribution)
> 
> 
> Average time is decreased by about 33%.
> 
> ### JMH results for testPutAll (size = 10000)
> 
> before patch:
> 
> 
> Result "org.openjdk.bench.java.util.concurrent.Maps.testConcurrentHashMapPutAll":
>   4315291.542 ±(99.9%) 336034.190 ns/op [Average]
>   (min, avg, max) = (3974688.194, 4315291.542, 4871772.209), stdev = 314326.589
>   CI (99.9%): [3979257.352, 4651325.731] (assumes normal distribution)
> 
> 
> after patch:
> 
> 
> Result "org.openjdk.bench.java.util.concurrent.Maps.testConcurrentHashMapPutAll":
>   3006955.723 ±(99.9%) 271757.969 ns/op [Average]
>   (min, avg, max) = (2801264.198, 3006955.723, 3553084.135), stdev = 254202.573
>   CI (99.9%): [2735197.754, 3278713.692] (assumes normal distribution)
> 
> 
> Average time is decreased about 30%.

This pull request has now been integrated.

Changeset: c432dc00
Author:    Joshua Cao <joshcao at amazon.com>
Committer: Volker Simonis <simonis at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/c432dc008bb3a2d3fe6b46617ad64a2999ae366c
Stats:     27 lines in 2 files changed: 24 ins; 1 del; 2 mod

8322149: ConcurrentHashMap smarter presizing for copy constructor and putAll

Reviewed-by: shade, simonis

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PR: https://git.openjdk.org/jdk/pull/17116