real time clock clock losing time

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Subject: Re: OpenServer 5 Clock Slippage
References: <3cbc3b19_1@nnrp1.news.uk.psi.net> <3CC00483.2CA712EF@strhold.it> <3ccd1e98_2@nnrp1.news.uk.psi.net> <3CCE476E.50C30F0B@strhold.it> 
From: spcecdt@deeptht.armory.com (John DuBois)
Date: 30 Apr 2002 22:04:08 GMT

In article <3CCE476E.50C30F0B@strhold.it>,  <r.zini@strhold.it> wrote:
>The "in kernel" clock is a "software" clock whereas the RTC is a hardware
>one; since the software clock is driven by software by means of $HZ interrupts,
>it's __likely__ that your system manages to miss some interrupts, perhaps
>due to heavy activity. As far as I know, some device drivers are able to
>protect critical pieces of code by "masking" (ie, disabling) interrupt so
>that they can execute without the risk of being "pre-empted" by the kernel.
>
>If your device driver is poorly written or the system is extremely busy/loaded,
>the software clock goes our of sync and this is the reason for setclk to be
>executed via cron.

Note that this is no longer true as of 5.0.6a - on machines that have a TSC
(which showed up in the Pentium II or thereabouts), the kernel clock uses the
TSC for timekeeping and so will not slip due to interrupt masking.  This also
allows for greater precision in timekeeping.

        John
-- 
John DuBois  spcecdt@armory.com  KC6QKZ/AE  http://www.armory.com/~spcecdt/
 

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