最後更新: 2023-12-27

介紹

hpet = High Precision Timer

Linux Default: TSC(Time Stamp Counter)

Enable & Use hpet

 * hpet need to enable in the BIOS

# Check if HPET is enabled

# timer_list: include both hardware and software timers

grep ^Clock /proc/timer_list

Clock Event Device: hpet
Clock Event Device: lapic
Clock Event Device: lapic

# If the HPET is to be used

ls -l /dev/hpet

crw------- 1 root root 10, 228 Nov  6 14:48 /dev/hpet

# Checking the current clock source

cat /sys/devices/system/clocksource/clocksource0/current_clocksource

tsc

# available clock sources

cat /sys/devices/system/clocksource/*/available_clocksource

tsc hpet acpi_pm

Settings

temporarily

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

kernel-parameters([X86-64] hpet,tsc)

clocksource=hpet    # Override the default clocksource

Hardware clock

Time Stamp Counter (TSC)

• the clock ticks at 1 GHz
• All 80x86 microprocessors include a CLK input pin, which receives the clock signal of an external oscillator.
• It is accessible through the TSC register which can be read by means of the rdtsc assembly instruction.

High Precision Event Timer (HPET)

• Basically the chip includes up to eight 32 bit or 64 bit independent counters.
• Each counter is driven by its own clock signal, whose frequency must be at least 10 MHz.
• The HPET registers allow the kernel to read and write the values of the counters and of the match registers, to program one-shot interrupts

ACPI Power Management Timer (ACPI PMT)

• included in almost all ACPI-based motherboards.
• a fixed frequency of roughly 3.58 MHz.
• May introduce issues with sub-millisecond resolution. (clock source of last resort only)