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我已经看到并阅读了有关为什么 System.nanoTime() 在某些操作系统上比其他操作系统慢的帖子,但是我从未看到任何东西可以解释我现在看到的差异。使用 JMH,我正在运行这个基准测试。(注意:它也使用 System.nanoTime() )

@Benchmark
public long systemNanoTime() {
    return System.nanoTime();
}

在 Windows 10 上,这需要大约 25 ns。

在 Centos 7、Linux 3.10 上,它被测量为大约 10293 ns。

这是在同一台机器上,Intel(R) Core(TM) i7-7820X CPU @ 3.60GHz

是否可以选择更改 JDK 获取系统时钟的方式?


编辑:根据托德提供的链接,似乎 tsc 不可用

# more /sys/devices/system/clocksource/clocksource0/*
::::::::::::::
/sys/devices/system/clocksource/clocksource0/available_clocksource
::::::::::::::
hpet acpi_pm 
::::::::::::::
/sys/devices/system/clocksource/clocksource0/current_clocksource
::::::::::::::
hpet

表演后

echo acpi_pm > /sys/devices/system/clocksource/clocksource0/current_clocksource

延迟有所改善,但仍然很差,延迟为 1,816 ns。

我试图找出是否可以将 TSC 时钟源添加到 Centos,但还没有运气。


编辑:进一步挖掘

# dmesg | grep -i tsc
[    0.000000] tsc: Detected 3600.000 MHz processor
[    0.058602] TSC deadline timer enabled
[    0.065868] TSC synchronization [CPU#0 -> CPU#1]:
[    0.065870] Measured 679995254538 cycles TSC warp between CPUs, turning off TSC clock.
[    0.065874] tsc: Marking TSC unstable due to check_tsc_sync_source failed
[  125.451480] Override clocksource tsc is not HRT compatible. Cannot switch while in HRT/NOHZ mode
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1 回答 1

6

根据@apangin 的建议,我按照这个页面添加了带有最新版本的centos 的替代存储库

http://elrepo.org/tiki/tiki-index.php

然后按照此处的进一步说明进行操作

https://www.tecmint.com/install-upgrade-kernel-version-in-centos-7/

安装并重启后

# $ dmesg | grep -i tsc
[    0.001000] tsc: Detected 3600.000 MHz processor
[    0.001000] [Firmware Bug]: TSC ADJUST: CPU0: -2100392876408 force to 0
[    0.046075] TSC deadline timer enabled
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU1: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU2: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU3: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU4: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU5: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU6: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU7: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU8: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU9: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU10: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU11: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU12: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU13: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU14: 0
[    0.001000] [Firmware Bug]: TSC ADJUST differs: Reference CPU0: -2100392876408 CPU15: 0
[    1.337843] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x6c1bafbc2ab, max_idle_ns: 881591058496 ns
[    2.353636] clocksource: Switched to clocksource tsc

暗示内核正在针对固件错误进行调整。

再次运行测试,我使用 System.nanoTime() 获得了 40 ns 的平均延迟,提高了 260 倍。这也意味着使用此度量的基准更有意义。

于 2017-08-22T07:28:18.850 回答