[Beowulf] precise synchronization of system clocks
Robert G. Brown
rgb at phy.duke.edu
Tue Sep 30 10:57:08 PDT 2008
On Tue, 30 Sep 2008, Lawrence Stewart wrote:
> I don 't think this is all that hard - once we got the cycle counters
> here at SiCortex, the rest of Linux behaves fairly well.
> The first trick is to get all the timer interrupts to happen at the same
> time. On
> our MIPS cores, this is easy by using an interrupt generated by a comparator
> against the cycle counter (architectural feature). I would hope there is
> similar in x86.
> Once the timer interrupts are synchronized, pretty much all background tasks
> syncronized. This even works for many device drivers that use the device
> interrupt to
> schedule a tasklet or top half.
> The next step is gang scheduling of kernel threads, so that all the, say,
> kswapd's system wide
> chose the same timer tick to wake up.
> The next step is gang scheduling of user threads, so that all the, say, cron
> wake up on the same timer tick.
> Our experience is that just aligning the timer interrupts helps a lot.
"At the same time" on what granularity? Timer is 1 KHz, right? So do
the timer interrupts have to be aligned to within a single clock tick, a
nsec, a usec? A usec is still quite small compared to a msec.
Robert G. Brown Phone(cell): 1-919-280-8443
Duke University Physics Dept, Box 90305
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