Re: SMP buildworld times / performance tests

[Warner Losh]

In message <[EMAIL PROTECTED]> Matthew Dillon writes:
:     The general problem with the timecounter is that not only is the hardware
:     indeterminant, but the timecounter structure itself is *NOT* MP safe,
:     at least not by my read of it.
: 
:     It also doesn't appear to be interrupt safe.  If a microtime() or 
:     getmicrotime() call is interrupted and the interrupting interrupt calls 
:     microtime(), it can corrupt the data returned by the first guy and
:     even corrupt the structure.

We've hacked the parallel port interrupt to be a fast one on one of
our boxes.  It is connected to the pps driver which calls getnanotime
to timestamp the pps pulse that came in.  We've seen, in carefully
plotting ntp data, that there are often (1 in a thousand) large
dropouts in the times reported.  They are in the neighborhood of the
clock tick.  Since ntp discards the outliers, this was a low priority
issue for us given the overall nature of that particular system.

At the time I took a look at it, and couldn't see how access to the
counter could be mp safe, but didn't have a lot of time to pursue it.

Warner


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