On Wed, 3 Jan 2024 21:29:57 GMT, Joshua Cao <d...@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%.
>
> Joshua Cao has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Cleanup benchmark
src/java.base/share/classes/java/util/concurrent/ConcurrentHashMap.java line
1088:
> 1086: public void putAll(Map<? extends K, ? extends V> m) {
> 1087: if (table != null) {
> 1088: tryPresize(m.size());
Shouldn't this be something like `tryPresize(this.size() + m.size())` to
accommodate for the worst case where there are no common keys in `this` map and
`m`?
Or do we intentionally try to be conservative with memory usage? But then
shouldn't we use at least `max(this.size(), m.size)`?
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/17116#discussion_r1444898430
