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

[David Holmes]

On Wed, 29 Mar 2023 11:28:53 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`

src/hotspot/share/runtime/javaThread.cpp line 1981:

> 1979: }
> 1980: 
> 1981: bool JavaThread::sleep(jlong millis, jint nanos) {

You don't need the overloads at this level - the incoming call should always have millis and nanos, even if nanos is zero.

src/hotspot/share/utilities/globalDefinitions.hpp line 351:

> 349:     millis = max_secs * MILLIUNITS;
> 350:   }
> 351:   return millis_to_nanos(millis) + nanos;

This can now exceed the max_secs bound though.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13225#discussion_r1152639527
PR Review Comment: https://git.openjdk.org/jdk/pull/13225#discussion_r1152640056