Re: [chrony-dev] [PATCH] MacOS X - Increase priority of chronyd process

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

Are you sure that there is a 1us offset? How do you measure that? Remember
that anything in the computer has latency and 1us is not that long a time. 
Ie, it takes a bit of work to measure that offset to that accuracy.

Note that the only time critical item is the reading of the system clock when
the interrupt comes in. And I have doubts that the interrupt handling software
is up to much less than 1us standard.


William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1     |      and Gravity       |  www.theory.physics.ubc.ca/

On Mon, 24 Aug 2015, Bryan Christianson wrote:

> Run chronyd with a 'nice' value of -15. Also run chronyd as a near real time process to prevent pre-empting of timing calculations and adjustments. This patch has the effect of removing an observed 1usec bias from the mean 'System time' offset.
> ---
> .../org.tuxfamily.chronyd.plist                    |  2 +
> sys_macosx.c                                       | 45 ++++++++++++++++++++++
> 2 files changed, 47 insertions(+)
>
> diff --git a/contrib/bryan_christianson_1/org.tuxfamily.chronyd.plist b/contrib/bryan_christianson_1/org.tuxfamily.chronyd.plist
> index 2bf42aa..58132fa 100644
> --- a/contrib/bryan_christianson_1/org.tuxfamily.chronyd.plist
> +++ b/contrib/bryan_christianson_1/org.tuxfamily.chronyd.plist
> @@ -13,6 +13,8 @@
> 		<string>-f</string>
> 		<string>/private/etc/chrony.d/chrony.conf</string>
> 	</array>
> +	<key>Nice</key>
> +	<integer>-15</integer>
> 	<key>KeepAlive</key>
> 	<true/>
> </dict>
> diff --git a/sys_macosx.c b/sys_macosx.c
> index 33da60d..f76f80c 100644
> --- a/sys_macosx.c
> +++ b/sys_macosx.c
> @@ -42,6 +42,10 @@
> #include <stdio.h>
> #include <signal.h>
>
> +#include <mach/mach.h>
> +#include <mach/mach_time.h>
> +#include <pthread.h>
> +
> #include "sys_macosx.h"
> #include "localp.h"
> #include "sched.h"
> @@ -85,6 +89,8 @@ static struct timeval Tdrift;
> /* minimum resolution of current_frequency */
> #define FREQUENCY_RES (1.0e-9)
>
> +#define NANOS_PER_MSEC (1000000ULL)
> +
> /* ================================================== */
>
> static void
> @@ -335,6 +341,43 @@ drift_removal_timeout(SCH_ArbitraryArgument not_used)
> }
>
> /* ================================================== */
> +/*
> +  Give chronyd real time priority so that time critical calculations
> +  are not pre-empted by the kernel.
> +*/
> +
> +static int
> +set_realtime(void)
> +{
> +  /* https://developer.apple.com/library/ios/technotes/tn2169/_index.html */
> +
> +  mach_timebase_info_data_t timebase_info;
> +  double clock2abs;
> +  thread_time_constraint_policy_data_t policy;
> +  int kr;
> +
> +  mach_timebase_info(&timebase_info);
> +  clock2abs = ((double)timebase_info.denom / (double)timebase_info.numer) * NANOS_PER_MSEC;
> +
> +  policy.period = 0;
> +  policy.computation = (uint32_t)(5 * clock2abs); /* 5 ms of work */
> +  policy.constraint = (uint32_t)(10 * clock2abs);
> +  policy.preemptible = 0;
> +
> +  kr = thread_policy_set(
> +          pthread_mach_thread_np(pthread_self()),
> +          THREAD_TIME_CONSTRAINT_POLICY,
> +          (thread_policy_t)&policy,
> +          THREAD_TIME_CONSTRAINT_POLICY_COUNT);
> +
> +  if (kr != KERN_SUCCESS) {
> +    LOG(LOGS_WARN, LOGF_SysMacOSX, "Cannot set real-time priority: %d", kr);
> +    return -1;
> +  }
> +  return 0;
> +}
> +
> +/* ================================================== */
>
> void
> SYS_MacOSX_Initialise(void)
> @@ -350,6 +393,8 @@ SYS_MacOSX_Initialise(void)
>
>   drift_removal_id = SCH_AddTimeoutByDelay(drift_removal_interval, drift_removal_timeout, NULL);
>   drift_removal_running = 1;
> +
> +  set_realtime();
> }
>
> /* ================================================== */
> --
> 2.3.2 (Apple Git-55)
>
>
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