如何以纳秒级分辨率快速获取当前时间？

Great question—getting sub-100ns timestamp resolution on Raspberry Pi for bit manipulation work is totally doable, but clock_gettime() is indeed too slow because of the system call overhead. Let’s break down your options from fastest to most involved:

1. User-Space: Directly Read ARM's System Counter (CNTVCT_EL0) #

The Raspberry Pi 3+ uses ARMv8 architecture, which includes a 64-bit monotonic system counter (CNTVCT_EL0) that ticks at a fixed hardware frequency (you can get this frequency from the CNTFRQ_EL0 register). Reading this register directly from user space skips all system call overhead, giving you timestamps in just a few nanoseconds—way faster than your 360ns clock_gettime() call.

Example Code #

Here’s how to implement this in C with inline assembly:

#include <stdint.h>

// Get the counter's tick frequency (run once at startup)
static inline uint64_t get_counter_frequency(void) {
    uint64_t freq;
    asm volatile("mrs %0, cntfrq_el0" : "=r"(freq));
    return freq;
}

// Read the current counter value
static inline uint64_t get_counter_ticks(void) {
    uint64_t ticks;
    asm volatile("mrs %0, cntvct_el0" : "=r"(ticks));
    return ticks;
}

// Convert ticks to nanoseconds
uint64_t get_high_res_ns(void) {
    static uint64_t freq = 0;
    if (freq == 0) {
        freq = get_counter_frequency();
    }
    uint64_t ticks = get_counter_ticks();
    // Multiply first to avoid floating-point operations (faster)
    return (ticks * 1000000000ULL) / freq;
}

Key Notes: #

• This counter is monotonic (never decreases) and won’t wrap around for ~5800 years at 1GHz, so it’s perfect for measuring time intervals.
• If you need to align this with wall-clock time, run a single clock_gettime(CLOCK_MONOTONIC, ...) at startup to calibrate an offset—after that, use the counter for all high-speed measurements.
• No root privileges required for this method.

2. Optimize clock_gettime() Usage #

If you need to stick to standard POSIX APIs, you can reduce clock_gettime() overhead by using the vDSO (Virtual Dynamic Shared Object) version. Modern glibc automatically uses vDSO for CLOCK_MONOTONIC and CLOCK_REALTIME, but you can explicitly prefer CLOCK_MONOTONIC_RAW (which skips NTP time adjustments) for slightly better performance:

struct timespec ts;
clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
uint64_t ns = (uint64_t)ts.tv_sec * 1000000000ULL + ts.tv_nsec;

This won’t be as fast as reading the hardware register directly, but it might cut your latency to ~50-100ns (depending on your Pi model).

3. Kernel Module: For Extreme Low-Latency Work #

If your bit manipulation requires even lower latency (e.g., disabling interrupts or running with highest priority), a kernel module is a viable option. Kernel-space time functions avoid user-to-kernel context switches and are optimized for speed.

Example Kernel Code #

Use ktime_get_ns(), which under the hood uses the same ARM system counter but with zero user-space overhead:

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/ktime.h>

static int __init high_res_timer_init(void) {
    uint64_t current_ns = ktime_get_ns();
    pr_info("High-res kernel time: %llu ns\n", current_ns);
    return 0;
}

static void __exit high_res_timer_exit(void) {
    pr_info("Module unloaded\n");
}

module_init(high_res_timer_init);
module_exit(high_res_timer_exit);
MODULE_LICENSE("GPL");

Pros & Cons: #

• Pros: Full control over scheduling (you can pin the kernel thread to a core, disable interrupts temporarily), and minimal latency for time reads.
• Cons: Higher development complexity, risk of crashing the system if your code has bugs, and limited access to user-space tools.

Final Recommendation #

For most bit manipulation tasks, directly reading the ARM system counter in user space is the best balance of speed, simplicity, and safety. Only consider a kernel module if you need interrupt control or absolute minimal latency that user space can’t provide.

内容的提问来源于stack exchange，提问作者user9592782