RFR: 8322149: ConcurrentHashMap copy constructor should not transfer from old table on presizing

[Aleksey Shipilev]

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%.

All right, good! I have comments about the benchmark:

src/java.base/share/classes/java/util/concurrent/ConcurrentHashMap.java line 851:

> 849:      */
> 850:     public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
> 851:         this(m.size(), LOAD_FACTOR, 1);

This looks like just `this(m.size())`, right? Not sure if we want to save an additional chained constructor call, though.

test/micro/org/openjdk/bench/java/util/concurrent/Maps.java line 83:

> 81:         for (int i = 0; i < nkeys; ++i) {
> 82:             staticMap.put(rng.next(), rng.next());
> 83:         }

Can just merge this loop with the one above, reusing `rng.next()` for both key and values for `staticMap`.

test/micro/org/openjdk/bench/java/util/concurrent/Maps.java line 122:

> 120:     public void testCopyConstructor() {
> 121:         ConcurrentHashMap<Integer, Integer> clone = new ConcurrentHashMap<>(staticMap);
> 122:         dummy = clone;

Is this for preventing dead-code elimination? If so, just do:


  return new ConcurrentHashMap<>(staticMap);

-------------

PR Review: https://git.openjdk.org/jdk/pull/17116#pullrequestreview-1802240545
PR Review Comment: https://git.openjdk.org/jdk/pull/17116#discussion_r1440523638
PR Review Comment: https://git.openjdk.org/jdk/pull/17116#discussion_r1440517305
PR Review Comment: https://git.openjdk.org/jdk/pull/17116#discussion_r1440515260