> 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%.
Joshua Cao has updated the pull request incrementally with one additional commit since the last revision: pass initial size to putAll benchmark map construction ------------- Changes: - all: https://git.openjdk.org/jdk/pull/17116/files - new: https://git.openjdk.org/jdk/pull/17116/files/3efa593d..2a965fa9 Webrevs: - full: https://webrevs.openjdk.org/?repo=jdk&pr=17116&range=03 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17116&range=02-03 Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod Patch: https://git.openjdk.org/jdk/pull/17116.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/17116/head:pull/17116 PR: https://git.openjdk.org/jdk/pull/17116
