[Xen-devel] RE: [Patch] Fix IDLE issue with sedf scheduler on IA64

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

>       To make ac_timer softirq triggered more precisely, another
> parameter tuned in the patch is the HZ, changed from 100(10ms) to
> 1024(1ms). Then the cpu_time of IDLE is decreased to about 0.09% from
> previous %1.

Hmmm... Are there other side effects from the HZ=32?  This patch
costs an extra ~1000 interrupts/second.  Since each of these interrupts
need to be handled in C code, the save/restore overhead is a large
fraction of the cost of a domain switch, thus increasing the total
Xen overhead (on _every_ application, even one which is CPU bound)
substantially.  Well, maybe not _substantially_ but probably on the
order of 0.1%.

Is there a better way (for ia64)?  I kind of like the solution
Keir and Ian imply... is it possible in context_switch to simply
"refuse" to switch to the idle domain?   E.g. if the idle domain
is the target of the switch, instead switch to domain0 (and
make it runnable)?

Dan

P.S. If this can be done in ia64-specific code, let's switch this
thread to xen-ia64-devel.  If not, we can later move the discussion
back to xen-devel.

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