[PATCH v5 09/11] trace: add high resolution timer function to debug performance issues
- From
Karsten Blees <karsten.blees@gmail.com>
- Date
- Jun 11, 2014, 08:01 UTC
- Message-ID
- <53980CBF.2060400@gmail.com>
- In-Reply-To
- <53980B83.9050409@gmail.com>
Add a getnanotime() function that returns nanoseconds since 01/01/1970 as unsigned 64-bit integer (i.e. overflows in july 2554). This is easier to work with than e.g. struct timeval or struct timespec. Basing the timer on the epoch allows using the results (div 10e9) with other time-related APIs.
To simplify adaption to different platforms, split the implementation into a common getnanotime() and a platform-specific highres_nanos() function.
The common getnanotime() function handles errors, falling back to gettimeofday() if highres_nanos() isn't implemented or doesn't work.
getnanotime() is also responsible for normalizing to the epoch. The offset to the system clock is calculated only once on initialization, i.e. manually setting the system clock has no impact on the timer (except if the fallback gettimeofday() is in use). Git processes are typically short lived, so we don't need to handle clock drift.
The highres_nanos() function returns monotonically increasing nanoseconds relative to some arbitrary point in time (e.g. system boot), or 0 on failure. Providing platform-specific implementations should be relatively easy, e.g. adapting to clock_gettime() as defined by the POSIX realtime extensions is seven lines of code.
This version includes highres_nanos() implementations for: * Linux: using clock_gettime(CLOCK_MONOTONIC) * Windows: using QueryPerformanceCounter()
Todo: * enable clock_gettime() on more platforms * add Mac OSX version, e.g. using mach_absolute_time + mach_timebase_info
Signed-off-by: Karsten Blees <blees@dcon.de> --- Makefile | 7 +++++ config.mak.uname | 1 + trace.c | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ trace.h | 1 + 4 files changed, 91 insertions(+)
diff --git a/Makefile b/Makefile index 07ea105..80f4390 100644 --- a/Makefile +++ b/Makefile @@ -340,6 +340,8 @@ all:: # # Define GMTIME_UNRELIABLE_ERRORS if your gmtime() function does not # return NULL when it receives a bogus time_t. +# +# Define HAVE_CLOCK_GETTIME if your platform has clock_gettime in librt. GIT-VERSION-FILE: FORCE @$(SHELL_PATH) ./GIT-VERSION-GEN @@ -1497,6 +1499,11 @@ ifdef GMTIME_UNRELIABLE_ERRORS BASIC_CFLAGS += -DGMTIME_UNRELIABLE_ERRORS endif +ifdef HAVE_CLOCK_GETTIME + BASIC_CFLAGS += -DHAVE_CLOCK_GETTIME + EXTLIBS += -lrt +endif + ifeq ($(TCLTK_PATH),) NO_TCLTK = NoThanks endif diff --git a/config.mak.uname b/config.mak.uname index 1ae675b..dad2618 100644 --- a/config.mak.uname +++ b/config.mak.uname @@ -34,6 +34,7 @@ ifeq ($(uname_S),Linux) HAVE_PATHS_H = YesPlease LIBC_CONTAINS_LIBINTL = YesPlease HAVE_DEV_TTY = YesPlease + HAVE_CLOCK_GETTIME = YesPlease endif ifeq ($(uname_S),GNU/kFreeBSD) HAVE_ALLOCA_H = YesPlease diff --git a/trace.c b/trace.c index 231e24a..4bd52f2 100644 --- a/trace.c +++ b/trace.c @@ -264,3 +264,85 @@ int trace_want(const char *key) return 0; return 1; } + +#ifdef HAVE_CLOCK_GETTIME + +static inline uint64_t highres_nanos(void) +{ + struct timespec ts; + if (clock_gettime(CLOCK_MONOTONIC, &ts)) + return 0; + return (uint64_t) ts.tv_sec * 1000000000 + ts.tv_nsec; +} + +#elif defined (GIT_WINDOWS_NATIVE) + +static inline uint64_t highres_nanos(void) +{ + static uint64_t high_ns, scaled_low_ns; + static int scale; + LARGE_INTEGER cnt; + + if (!scale) { + if (!QueryPerformanceFrequency(&cnt)) + return 0; + + /* high_ns = number of ns per cnt.HighPart */ + high_ns = (1000000000LL << 32) / (uint64_t) cnt.QuadPart; + + /* + * Number of ns per cnt.LowPart is 10^9 / frequency (or + * high_ns >> 32). For maximum precision, we scale this factor + * so that it just fits within 32 bit (i.e. won't overflow if + * multiplied with cnt.LowPart). + */ + scaled_low_ns = high_ns; + scale = 32; + while (scaled_low_ns >= 0x100000000LL) { + scaled_low_ns >>= 1; + scale--; + } + } + + /* if QPF worked on initialization, we expect QPC to work as well */ + QueryPerformanceCounter(&cnt); + + return (high_ns * cnt.HighPart) + + ((scaled_low_ns * cnt.LowPart) >> scale); +} + +#else +# define highres_nanos() 0 +#endif + +static inline uint64_t gettimeofday_nanos(void) +{ + struct timeval tv; + gettimeofday(&tv, NULL); + return (uint64_t) tv.tv_sec * 1000000000 + tv.tv_usec * 1000; +} + +/* + * Returns nanoseconds since the epoch (01/01/1970), for performance tracing + * (i.e. favoring high precision over wall clock time accuracy). + */ +inline uint64_t getnanotime(void) +{ + static uint64_t offset; + if (offset > 1) { + /* initialization succeeded, return offset + high res time */ + return offset + highres_nanos(); + } else if (offset == 1) { + /* initialization failed, fall back to gettimeofday */ + return gettimeofday_nanos(); + } else { + /* initialize offset if high resolution timer works */ + uint64_t now = gettimeofday_nanos(); + uint64_t highres = highres_nanos(); + if (highres) + offset = now - highres; + else + offset = 1; + return now; + } +} diff --git a/trace.h b/trace.h index 916dc2b..c4964aa 100644 --- a/trace.h +++ b/trace.h @@ -6,6 +6,7 @@ extern void trace_repo_setup(const char *prefix); extern int trace_want(const char *key); +extern uint64_t getnanotime(void); #ifndef HAVE_VARIADIC_MACROS
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