----- Original Message -----
> From: "Mathieu Desnoyers" <[email protected]>
> To: [email protected]
> Cc: [email protected], "Mathieu Desnoyers"
> <[email protected]>
> Sent: Monday, January 27, 2014 2:28:36 PM
> Subject: [PATCH lttng-tools] Fix: measure UST clock offset with best sample
>
> Fixes #729
>
> Signed-off-by: Mathieu Desnoyers <[email protected]>
> ---
> src/bin/lttng-sessiond/ust-metadata.c | 57
> ++++++++++++++++++++++++++-------
> 1 file changed, 46 insertions(+), 11 deletions(-)
>
> diff --git a/src/bin/lttng-sessiond/ust-metadata.c
> b/src/bin/lttng-sessiond/ust-metadata.c
> index b0f83d2..55d00da 100644
> --- a/src/bin/lttng-sessiond/ust-metadata.c
> +++ b/src/bin/lttng-sessiond/ust-metadata.c
> @@ -24,6 +24,7 @@
> #include <string.h>
> #include <stdarg.h>
> #include <stdio.h>
> +#include <stdlib.h>
Hrm, I forgot to remove this include from the previous patch.
Will wait for comments before re-send.
Thanks,
Mathieu
> #include <limits.h>
> #include <unistd.h>
> #include <inttypes.h>
> @@ -37,6 +38,13 @@
> #define max_t(type, a, b) ((type) ((a) > (b) ? (a) : (b)))
> #endif
>
> +#define NR_CLOCK_OFFSET_SAMPLES 10
> +
> +struct offset_sample {
> + uint64_t offset; /* correlation offset */
> + uint64_t measure_delta; /* lower is better */
> +};
> +
> static inline
> int fls(unsigned int x)
> {
> @@ -490,31 +498,58 @@ int _lttng_event_header_declare(struct
> ust_registry_session *session)
> );
> }
>
> -/*
> - * Approximation of NTP time of day to clock monotonic correlation,
> - * taken at start of trace.
> - * Yes, this is only an approximation. Yes, we can (and will) do better
> - * in future versions.
> - */
> static
> -uint64_t measure_clock_offset(void)
> +int measure_single_clock_offset(struct offset_sample *sample)
> {
> - uint64_t offset, monotonic[2], realtime;
> + uint64_t offset, monotonic[2], measure_delta, realtime;
> struct timespec rts = { 0, 0 };
> int ret;
>
> monotonic[0] = trace_clock_read64();
> ret = clock_gettime(CLOCK_REALTIME, &rts);
> - if (ret < 0)
> - return 0;
> + if (ret < 0) {
> + return ret;
> + }
> monotonic[1] = trace_clock_read64();
> + measure_delta = monotonic[1] - monotonic[0];
> + if (measure_delta > sample->measure_delta) {
> + /*
> + * Discard value if it took longer to read than the best
> + * sample so far.
> + */
> + return 0;
> + }
> offset = (monotonic[0] + monotonic[1]) >> 1;
> realtime = (uint64_t) rts.tv_sec * 1000000000ULL;
> realtime += rts.tv_nsec;
> offset = realtime - offset;
> - return offset;
> + sample->offset = offset;
> + sample->measure_delta = measure_delta;
> + return 0;
> }
>
> +/*
> + * Approximation of NTP time of day to clock monotonic correlation,
> + * taken at start of trace.
> + * Take the median of many measurements to remove uncertainty caused by
> + * preemption.
> + */
> +static
> +uint64_t measure_clock_offset(void)
> +{
> + int i;
> + struct offset_sample offset_best_sample = {
> + .offset = 0,
> + .measure_delta = UINT64_MAX,
> + };
> +
> + for (i = 0; i < NR_CLOCK_OFFSET_SAMPLES; i++) {
> + if (measure_single_clock_offset(&offset_best_sample)) {
> + return 0;
> + }
> + }
> + return offset_best_sample.offset;
> +}
>
> /*
> * Should be called with session registry mutex held.
> --
> 1.7.10.4
>
>
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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