RFR: 8278241: Implement JVM SpinPause on linux-aarch64 [v2]

[Stuart Monteith]

On Mon, 13 Dec 2021 15:56:02 GMT, Evgeny Astigeevich <duke at openjdk.java.net> wrote:

>> This JVM SpinPause uses a spin wait stub. The stub is generated based on the `SpinWait` description which is defined with `OnSpinWaitInst`/`OnSpinWaitInstCount` options.  The `SpinWait` provides the description of the instruction and the instruction count.
>> 
>> The `SpinWait` description is also used for the `_onSpinWait()` intrinsic. We don't have use cases when we need different implementations for the `_onSpinWait()` intrinsic and JVM SpinPause.
>> 
>> Testing results for fastdebug and release builds:
>> - `gtest`: Passed
>> - `tier1`...`tier4`: Passed
>> - `hotspot/jtreg/runtime/Thread/TestSpinPause.java`: Passed
>> 
>> JVM SpinPause is used for the synchronised statements and can benchmarked with `org.openjdk.bench.vm.lang.LockUnlock.testContendedLock`.
>> 
>> Benchmarking results (number of samples per an experiment: 150) for Graviton2 (Neoverse N1), 1 ISB instruction:
>> 
>> 
>> 
>> +-----------+-------------------+------------+-----------+-----------+----------+---------+
>> | CPU cores | Contended threads | Base ns/op |   Error   |    New    |  Error   |  Diff   |
>> +-----------+-------------------+------------+-----------+-----------+----------+---------+
>> |         8 |                64 |  10007.213 | ±910.911  |  8527.346 | ±377.242 | -14.79% |
>> |        16 |                64 |  10274.935 | ±880.568  |  8310.433 | ±326.845 | -19.12% |
>> |        32 |                64 |  12231.947 | ±1525.364 |  9205.941 | ±394.409 | -24.74% |
>> |        64 |                64 |    9929.49 | ±586.074  | 10488.695 | ±570.458 | 5.63%   |
>> |        64 |                32 |   5605.119 | ±629.340  |  5023.882 | ±230.639 | -10.37% |
>> |        64 |                16 |   2817.346 | ±263.696  |  2367.528 | ±94.158  | -15.97% |
>> |        64 |                 2 |    870.389 | ±530.579  |   464.395 | ±126.260 | -46.65% |
>> +-----------+-------------------+------------+-----------+-----------+----------+---------+
>
> Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Reimplement JVM SpinPause with using stub code

src/hotspot/os_cpu/linux_aarch64/os_linux_aarch64.cpp line 387:

> 385:     if (VM_Version::spin_wait_desc().inst() == SpinWait::NONE) {
> 386:       return 0;
> 387:     }

Would it be safe and more efficient to test func for NULL?

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PR: https://git.openjdk.java.net/jdk/pull/6803