diff options
Diffstat (limited to 'src/lib/time/compat_time.h')
| -rw-r--r-- | src/lib/time/compat_time.h | 146 |
1 files changed, 144 insertions, 2 deletions
diff --git a/src/lib/time/compat_time.h b/src/lib/time/compat_time.h index 480d426ac7..8c7661d7cb 100644 --- a/src/lib/time/compat_time.h +++ b/src/lib/time/compat_time.h @@ -15,6 +15,120 @@ * of tens of milliseconds. */ +/* Q: When should I use monotonic time? + * + * A: If you need a time that never decreases, use monotonic time. If you need + * to send a time to a user or another process, or store a time, use the + * wall-clock time. + * + * Q: Should you use monotime or monotime_coarse as your source? + * + * A: Generally, you get better precision with monotime, but better + * performance with monotime_coarse. + * + * Q: What is a "monotonic" time, exactly? + * + * A: Monotonic times are strictly non-decreasing. The difference between any + * previous monotonic time, and the current monotonic time, is always greater + * than *or equal to* zero. + * Zero deltas happen more often: + * - on Windows (due to an OS bug), + * - when using monotime_coarse, or on systems with low-resolution timers, + * - on platforms where we emulate monotonic time using wall-clock time, and + * - when using time units that are larger than nanoseconds (due to + * truncation on division). + * + * Q: Should you use monotime_t or monotime_coarse_t directly? Should you use + * usec? msec? "stamp units?" + * + * A: Using monotime_t and monotime_coarse_t directly is most time-efficient, + * since no conversion needs to happen. But they can potentially use more + * memory than you would need for a usec/msec/"stamp unit" count. + * + * Converting to usec or msec on some platforms, and working with them in + * general, creates a risk of doing a 64-bit division. 64-bit division is + * expensive on 32-bit platforms, which still do exist. + * + * The "stamp unit" type is designed to give a type that is cheap to convert + * from monotime_coarse, has resolution of about 1-2ms, and fits nicely in a + * 32-bit integer. Its downside is that it does not correspond directly + * to a natural unit of time. + * + * There is not much point in using "coarse usec" or "coarse nsec", since the + * current coarse monotime implementations give you on the order of + * milliseconds of precision. + * + * Q: So, what backends is monotime_coarse using? + * + * A: Generally speaking, it uses "whatever monotonic-ish time implemenation + * does not require a context switch." The various implementations provide + * this by having a view of the current time in a read-only memory page that + * is updated with a frequency corresponding to the kernel's tick count. + * + * On Windows, monotime_coarse uses GetCount64() [or GetTickCount() on + * obsolete systems]. MSDN claims that the resolution is "typically in the + * range of 10-16 msec", but it has said that for years. Storing + * monotime_coarse_t uses 8 bytes. + * + * On OSX/iOS, monotime_coarse uses uses mach_approximate_time() where + * available, and falls back to regular monotime. The precision is not + * documented, but the implementation is open-source: it reads from a page + * that the kernel updates. Storing monotime_coarse_t uses 8 bytes. + * + * On unixy systems, monotime_coarse uses clock_gettime() with + * CLOCK_MONOTONIC_COARSE where available, and falls back to CLOCK_MONOTONIC. + * It typically uses vdso tricks to read from a page that the kernel updates. + * Its precision fixed, but you can get it with clock_getres(): on my Linux + * desktop, it claims to be 1 msec, but it will depend on the system HZ + * setting. Storing monotime_coarse_t uses 16 bytes. + * + * [TODO: Try CLOCK_MONOTONIC_FAST on foobsd.] + * + * Q: What backends is regular monotonic time using? + * + * A: In general, regular monotime uses something that requires a system call. + * On platforms where system calls are cheap, you win! Otherwise, you lose. + * + * On Windows, monotonic time uses QuereyPerformanceCounter. Storing + * monotime_t costs 8 bytes. + * + * On OSX/Apple, monotonic time uses mach_absolute_time. Storing + * monotime_t costs 8 bytes. + * + * On unixy systems, monotonic time uses CLOCK_MONOTONIC. Storing + * monotime_t costs 16 bytes. + * + * Q: Tell me about the costs of converting to a 64-bit nsec, usec, or msec + * count. + * + * A: Windows, coarse: Cheap, since it's all multiplication. + * + * Windows, precise: Expensive on 32-bit: it needs 64-bit division. + * + * Apple, all: Expensive on 32-bit: it needs 64-bit division. + * + * Unixy, all: Fairly cheap, since the only division required is dividing + * tv_nsec 1000, and nanoseconds-per-second fits in a 32-bit value. + * + * All, "timestamp units": Cheap everywhere: it never divides. + * + * Q: This is only somewhat related, but how much precision could I hope for + * from a libevent time? + * + * A: Actually, it's _very_ related if you're timing in order to have a + * timeout happen. + * + * On Windows, it uses select: you could in theory have a microsecond + * resolution, but it usually isn't that accurate. + * + * On OSX, iOS, and BSD, you have kqueue: You could in theory have a nanosecond + * resolution, but it usually isn't that accurate. + * + * On Linux, you have epoll: It has a millisecond resolution. Some recent + * Libevents can also use timerfd for higher resolution if + * EVENT_BASE_FLAG_PRECISE_TIMER is set: Tor doesn't set that flag. + */ + #ifndef TOR_COMPAT_TIME_H #define TOR_COMPAT_TIME_H @@ -86,26 +200,36 @@ void monotime_init(void); void monotime_get(monotime_t *out); /** * Return the number of nanoseconds between <b>start</b> and <b>end</b>. + * The returned value may be equal to zero. */ int64_t monotime_diff_nsec(const monotime_t *start, const monotime_t *end); /** * Return the number of microseconds between <b>start</b> and <b>end</b>. + * The returned value may be equal to zero. + * Fractional units are truncated, not rounded. */ int64_t monotime_diff_usec(const monotime_t *start, const monotime_t *end); /** * Return the number of milliseconds between <b>start</b> and <b>end</b>. + * The returned value may be equal to zero. + * Fractional units are truncated, not rounded. */ int64_t monotime_diff_msec(const monotime_t *start, const monotime_t *end); /** * Return the number of nanoseconds since the timer system was initialized. + * The returned value may be equal to zero. */ uint64_t monotime_absolute_nsec(void); /** * Return the number of microseconds since the timer system was initialized. + * The returned value may be equal to zero. + * Fractional units are truncated, not rounded. */ -uint64_t monotime_absolute_usec(void); +MOCK_DECL(uint64_t, monotime_absolute_usec,(void)); /** * Return the number of milliseconds since the timer system was initialized. + * The returned value may be equal to zero. + * Fractional units are truncated, not rounded. */ uint64_t monotime_absolute_msec(void); @@ -129,6 +253,9 @@ void monotime_add_msec(monotime_t *out, const monotime_t *val, uint32_t msec); * Set <b>out</b> to the current coarse time. */ void monotime_coarse_get(monotime_coarse_t *out); +/** + * Like monotime_absolute_*(), but faster on some platforms. + */ uint64_t monotime_coarse_absolute_nsec(void); uint64_t monotime_coarse_absolute_usec(void); uint64_t monotime_coarse_absolute_msec(void); @@ -152,18 +279,27 @@ uint32_t monotime_coarse_to_stamp(const monotime_coarse_t *t); /** * Convert a difference, expressed in the units of monotime_coarse_to_stamp, * into an approximate number of milliseconds. + * + * The returned value may be equal to zero. + * Fractional units are truncated, not rounded. */ uint64_t monotime_coarse_stamp_units_to_approx_msec(uint64_t units); uint64_t monotime_msec_to_approx_coarse_stamp_units(uint64_t msec); uint32_t monotime_coarse_get_stamp(void); #if defined(MONOTIME_COARSE_TYPE_IS_DIFFERENT) +/** + * Like monotime_diff_*(), but faster on some platforms. + */ int64_t monotime_coarse_diff_nsec(const monotime_coarse_t *start, const monotime_coarse_t *end); int64_t monotime_coarse_diff_usec(const monotime_coarse_t *start, const monotime_coarse_t *end); int64_t monotime_coarse_diff_msec(const monotime_coarse_t *start, const monotime_coarse_t *end); +/** + * Like monotime_*(), but faster on some platforms. + */ void monotime_coarse_zero(monotime_coarse_t *out); int monotime_coarse_is_zero(const monotime_coarse_t *val); void monotime_coarse_add_msec(monotime_coarse_t *out, @@ -182,6 +318,9 @@ void monotime_coarse_add_msec(monotime_coarse_t *out, * * Requires that the difference fit into an int32_t; not for use with * large time differences. + * + * The returned value may be equal to zero. + * Fractional units are truncated, not rounded. */ int32_t monotime_coarse_diff_msec32_(const monotime_coarse_t *start, const monotime_coarse_t *end); @@ -191,6 +330,9 @@ int32_t monotime_coarse_diff_msec32_(const monotime_coarse_t *start, * * Requires that the difference fit into an int32_t; not for use with * large time differences. + * + * The returned value may be equal to zero. + * Fractional units are truncated, not rounded. */ static inline int32_t monotime_coarse_diff_msec32(const monotime_coarse_t *start, @@ -202,7 +344,7 @@ monotime_coarse_diff_msec32(const monotime_coarse_t *start, #else #define USING_32BIT_MSEC_HACK return monotime_coarse_diff_msec32_(start, end); -#endif +#endif /* SIZEOF_VOID_P == 8 */ } #ifdef TOR_UNIT_TESTS |