• Rolf Fokkens's avatar
    [PATCH] USER_HZ & NTP problems · 3843e047
    Rolf Fokkens authored
    I've been playing with different HZ values in the 2.4 kernel for a while
    now, and apparantly Linus also has decided to introduce a USER_HZ
    constant (I used CLOCKS_PER_SEC) while raising the HZ value on x86 to
    1000.
    
    On x86 timekeeping has shown to be relative fragile when raising HZ (OK,
    I tried HZ=2048 which is quite high) because of the way the interrupt
    timer is configured to fire HZ times each second.  This is done by
    configuring a divisor in the timer chip (LATCH) which divides a certain
    clock (1193180) and makes the chip fire interrupts at the resulting
    frequency.
    
    Now comes the catch: NTP requires a clock accuracy of 500 ppm.  For some
    HZ values the clock is not accurate enough to meet this requirement,
    hence NTP won't work well.
    
    An example HZ value is 1020 which exceeds the 500 ppm requirement.  In
    this case the best approximation is 1019.8 Hz.  the xtime.tv_usec value
    is raised with a value of 980 each tick which means that after one
    second the tv_usec value has increased with 999404 (should be 1000000)
    which is an accuracy of 596 ppm.
    
    Some more examples:
    	  HZ Accuracy (ppm)
    	---- --------------
    	 100             17
    	1000            151
    	1024            632
    	2000            687
    	2008            343
    	2011             18
    	2048           1249
    
    What I've been doing is replace tv_usec by tv_nsec, meaning xtime is now
    a timespec instead of a timeval.  This allows the accuracy to be
    improved by a factor of 1000 for any (well ...  any?) HZ value. 
    
    Of course all kinds of calculations had te be improved as well.  The
    ACTHZ constantant is introduced to approximate the actual HZ value, it's
    used to do some approximations of other related values. 
    3843e047
udfdecl.h 8.04 KB