SCO Unix, ALR FlexCACHE losing time

[Tim Wright]

In <1991Jan2.221527.15181 at gsm001.uucp> gsm at gsm001.uucp (Geoffrey S. Mendelson) writes:


>in Message-ID: <1991Jan01.162400.6155 at litwin.com>
>Dr. Victor L. Rice writes:
>>	      
>>What gives ??? Why am I losing over a second a day ??
>>-- 
...
>2:  When UNIX (and MS-DOS) are booted, the read the battery backed up clock.
>    From then on they update the time on each "clock tick interupt".
>    Most device drivers, especially disk, turn off interupts while they 
>    are running.  

>    The clock will be "off" 1/60 of a second until the interupt gets processed.
>    Since disk drivers don't want to be interupted, they turn off interupts
>    during transfers.  If for some reason they are busy for more than 1/60 of
>    a second you loose any clock ticks after the first.

>    If you have a tape or SCSI driver that is hit very hard you may see this.
>    Also a serial card driver may block interupts, but not likely for that 
>    long a time.  
>      
Is this really the case? How often do the above drivers do an 'splhi()' which
would lock out the clock. I can't imagine it happens that much. The AT&T 3B15
had a bug where the disk buffer cache code locked out the clock and it really
knocked the time out of whack when busy. From memory, v7-style block drivers
lock the buffer cache with spl6, which doesn't affect the clock. I suppose
a driver doing "programmed-io" might want to lock out all interrupts but I
can't see that squirting 512 bytes in a loop takes that long on a modern
system ! Anybody care to comment/correct me.
>Also a note on accuracy:
> 
>    1%  would be 864 seconds a day or 14 minutes 24 seconds
>   .1%  would be 86  seconds a day or  1 minute  26 seconds
>   .01% would be 8.6 seconds a day
>   1 second a day is 1/86400 or 1 in almost 1 part in one hundred thousand.

>How many scientific instruments can boast that accuracy?
Well my casio wristwatch (quite cheap) is GUARANTEED accurate to 15 seconds a
month or ~ 1/172800. I fail to see why the RTC-chip manufacturers can't make
one at least as good (except for pricing considerations :-)

Tim
--
Tim Wright, Dell Computer Corp. (UK) | Email address
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"What's the problem? You've got an IQ of six thousand, haven't you?"