> Robert de Vries <rhdv@rhdv.cistron.nl> writes:
>
>> I disagree, because the system call overhead takes more time than the
>> execution of a switch statement.
>
> You can add some clocks on user level but the handling of interrupts
> for timers et cannot efficiently be done at user level.
You are absolutely right, but nobody says that you have to implement both
clock and timer functionality for each CLOCK_<type>.
So some CLOCKs have only clock functionality, some only timer, and the
rest has both.
On SGI IRIX 6.5 you have for instance three clocks:
CLOCK_REALTIME (all clock & timer functions supported)
CLOCK_SGI_CYCLE (clock_gettime() and clock_getres() only)
CLOCK_SGI_FAST (timer_create() & clock_getres() only)
For each of these clocks there can be a mix of how and where each function
is implemented. The decision of where the implementation of each function
can be done best is made by the kernel side developer (=Robert) and the
C library developer (=Ulrich) in close cooperation.
I am a performance freak, as I make real-time systems. So I am prepared to
trade some simplicity of implementation for performance.
Robert
-- Robert H. de Vries PO/SIM Fokker Space B.V. e-mail: R.de.Vries@fokkerspace.nl tel: (+31)71-5245464 fax: (+31)71-5245498
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