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kernel: kconfig: changing recommendations for SYS_CLOCK_TICKS_PER_SEC

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Documentation for kconfig SYS_CLOCK_TICKS_PER_SEC has some outdated
recommendations. Changing them to align with other documentation
under kernel timing.

Fixes: #25482

Signed-off-by: Spoorthy Priya Yerabolu <spoorthy.priya.yerabolu@intel.com>
This commit is contained in:
Spoorthy Priya Yerabolu 2020-09-18 00:45:48 -07:00 • committed by Anas Nashif
commit e9ef078a11
1 changed files with 11 additions and 17 deletions
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kernel/Kconfig
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@ -537,24 +537,18 @@ config SYS_CLOCK_TICKS_PER_SEC
help
This option specifies the nominal frequency of the system clock in Hz.
Depending on the choice made, an amount of possibly expensive math must
occur when converting ticks to milliseconds and vice-versa. Some values
are optimized, and yield significantly less math.
For asynchronous timekeeping, the kernel defines a "ticks" concept. A
"tick" is the internal count in which the kernel does all its internal
uptime and timeout bookeeping. Interrupts are expected to be delivered
on tick boundaries to the extent practical, and no fractional ticks
are tracked.
The optimal values from a computational point-of-view are 1000, 500,
250 and 125, since in these cases there is either no computation
required, or it is all done via bit-shifting. These also give a
granularity from 1ms to 8ms.
Other good values are 100, 50, 25, 20 and 10. In this case, some math
is required but is minimized. These are also values that necessitate a
reduced number of clock interrupts per second, at the cost of
granularity (10ms to 100ms).
All other values require some extensive 64-bit math, and in some
configurations even require calls to compiler built-in functions, and
can require a non-trivial extra amount of stack space (e.g. around 80
bytes on x86).
The choice of tick rate is configurable by this option. Also the number
of cycles per tick should be chosen so that 1 millisecond is exactly
represented by an integral number of ticks. Defaults on most hardware
platforms (ones that support setting arbitrary interrupt timeouts) are
expected to be in the range of 10 kHz, with software emulation
platforms and legacy drivers using a more traditional 100 Hz value.
Note that when available and enabled, in "tickless" mode
this config variable specifies the minimum available timing