问题
I have seen and read posts on why System.nanoTime() is slower on some OSes than others, however I have never seen anything to explain the difference I am seeing now. Using JMH, I am running this benchmark. (Note: it uses System.nanoTime() as well)
@Benchmark
public long systemNanoTime() {
return System.nanoTime();
}
On Windows 10, this takes ~25 ns.
On Centos 7, Linux 3.10 it is measured as taking ~10293 ns.
This is on the same machine, Intel(R) Core(TM) i7-7820X CPU @ 3.60GHz
Is there an option to change the way the JDK gets the system clock?
EDIT: Based on the link provided by Todd, it appear that tsc is not available
# more /sys/devices/system/clocksource/clocksource0/*
::::::::::::::
/sys/devices/system/clocksource/clocksource0/available_clocksource
::::::::::::::
hpet acpi_pm
::::::::::::::
/sys/devices/system/clocksource/clocksource0/current_clocksource
::::::::::::::
hpet
after performing
echo acpi_pm > /sys/devices/system/clocksource/clocksource0/current_clocksource
The latency improved, but is still poor with a latency of 1,816 ns.
I have tried to find out if the TSC clock source can be added to Centos, but not luck yet.
EDIT: Digging a little further
# 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
回答1:
Based on @apangin's suggestion I followed this page on adding the alternative repository for centos with the latest version
http://elrepo.org/tiki/tiki-index.php
and then followed the further instructions here
https://www.tecmint.com/install-upgrade-kernel-version-in-centos-7/
after installing and rebooting
# $ 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
suggesting the kernel is adjusting for a firmware bug.
Running the test again, I get an average latency of 40 ns using System.nanoTime() which is a 260 fold improvement. It also means that benchmarks using this measure are more meaningful.
来源：https://stackoverflow.com/questions/45803565/centos-7-400x-slower-for-system-nanotime-than-windows