RFR: 8305092: Improve Thread.sleep(millis, nanos) for sub-millisecond granularity [v7]

[David Holmes]

On Wed, 19 Apr 2023 19:55:38 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Java API has the `Thread.sleep(millis, nanos)` method exposed to users. The documentation for that method clearly says the precision and accuracy are dependent on the underlying system behavior. However, it always rounds up `nanos` to 1ms when doing the actual sleep. This means users cannot do the micro-second precision sleeps, even when the underlying platform allows it. Sub-millisecond sleeps are useful to build interesting primitives, like the rate limiters that run with >1000 RPS.
>> 
>> When faced with this, some users reach for more awkward APIs like `java.util.concurrent.locks.LockSupport.parkNanos`. The use of that API for sleeps is not in line with its intent, and while it "seems to work", it might have interesting interactions with other uses of `LockSupport`. Additionally, these "sleeps" are no longer visible to monitoring tools as "normal sleeps", e.g. as `Thread.sleep` events. Therefore, it would be prudent to improve current `Thread.sleep(millis, nanos)` for sub-millisecond granularity. 
>> 
>> Fortunately, the underlying code is almost ready for this, at least on POSIX side. I skipped Windows paths, because its timers are still no good. Note that on both Linux and MacOS timers oversleep by about 50us. I have a few ideas how to improve the accuracy for them, which would be a topic for a separate PR.
>> 
>> Additional testing:
>>   - [x] New regression test
>>   - [x] New benchmark
>>   - [x] Linux x86_64 `tier1`
>>   - [x] Linux AArch64 `tier1`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Adjust test times

Looking much cleaner/simpler. A couple more comments below. Thanks.

src/hotspot/os/windows/os_windows.cpp line 5263:

> 5261:   if (nanos_left > 0) {
> 5262:     millis++;
> 5263:   }

You could simplify this to:

if (nanos > millis * NANOSECS_PER_MILLISEC) {
  millis++;
}

src/hotspot/os/windows/park_windows.hpp line 53:

> 51:     void park () ;
> 52:     void unpark () ;
> 53:     int  park(jlong millis);

While you are here could you get rid of the spaces on park/unpark before the parentheses - thanks.

src/hotspot/share/runtime/javaThread.cpp line 1983:

> 1981: }
> 1982: 
> 1983: bool JavaThread::sleep(jlong millis) {

Perhaps a comment:

// Internal convenience function for millisecond resolution sleeps.

src/java.base/share/classes/java/lang/Thread.java line 1:

> 1: /*

Why doesn't `Thread.sleep(long millis)` simply call `Thread.sleep(millis, 0)` instead of duplicating most of the code?

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PR Review: https://git.openjdk.org/jdk/pull/13225#pullrequestreview-1393107848
PR Review Comment: https://git.openjdk.org/jdk/pull/13225#discussion_r1172011960
PR Review Comment: https://git.openjdk.org/jdk/pull/13225#discussion_r1172013223
PR Review Comment: https://git.openjdk.org/jdk/pull/13225#discussion_r1172014393
PR Review Comment: https://git.openjdk.org/jdk/pull/13225#discussion_r1172017034