On Tue, 20 May 2025 17:12:08 GMT, kieran-farrell <d...@openjdk.org> wrote:
>> With the recent approval of UUIDv7 >> (https://datatracker.ietf.org/doc/rfc9562/), this PR aims to add a new >> static method UUID.timestampUUID() which constructs and returns a UUID in >> support of the new time generated UUID version. >> >> The specification requires embedding the current timestamp in milliseconds >> into the first bits 0–47. The version number in bits 48–51, bits 52–63 are >> available for sub-millisecond precision or for pseudorandom data. The >> variant is set in bits 64–65. The remaining bits 66–127 are free to use for >> more pseudorandom data or to employ a counter based approach for increased >> time percision >> (https://www.rfc-editor.org/rfc/rfc9562.html#name-uuid-version-7). >> >> The choice of implementation comes down to balancing the sensitivity level >> of being able to distingush UUIDs created below <1ms apart with performance. >> A test simulating a high-concurrency environment with 4 threads generating >> 10000 UUIDv7 values in parallel to measure the collision rate of each >> implementation (the amount of times the time based portion of the UUID was >> not unique and entries could not distinguished by time) yeilded the >> following results for each implemtation: >> >> >> - random-byte-only - 99.8% >> - higher-precision - 3.5% >> - counter-based - 0% >> >> >> Performance tests show a decrease in performance as expected with the >> counter based implementation due to the introduction of synchronization: >> >> - random-byte-only 143.487 ± 10.932 ns/op >> - higher-precision 149.651 ± 8.438 ns/op >> - counter-based 245.036 ± 2.943 ns/op >> >> The best balance here might be to employ a higher-precision implementation >> as the large increase in time sensitivity comes at a very slight performance >> cost. > > kieran-farrell has updated the pull request incrementally with one additional > commit since the last revision: > > Update src/java.base/share/classes/java/util/UUID.java > > Co-authored-by: Andrey Turbanov <turban...@gmail.com> src/java.base/share/classes/java/util/UUID.java line 200: > 198: public static UUID timestampUUID() { > 199: long msTime = System.currentTimeMillis(); > 200: long nsTime = System.nanoTime(); I is my understanding that `System#nanoTime()` does not provide the nanosecond fraction and is not guaranteed to have nanosecond resolution. You'd have to look at `VM#getNanoTimeAdjustment(long)` to get the nanotime adjustment. However the resolution of this is OS dependent, usually microseconds, likely 10 microseconds on Windows. src/java.base/share/classes/java/util/UUID.java line 203: > 201: SecureRandom ng = Holder.numberGenerator; > 202: byte[] randomBytes = new byte[16]; > 203: ng.nextBytes(randomBytes); It's a bit unfortunate that `SecureRandom#nextBytes(byte[])` does not allow us to pass an offset and length. Here we generate 16 bytes of random data but throw the first 8 bytes away. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2098704776 PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2098708696
