On Fri, 2007-11-09 at 02:44 -0800, Stephane Eranian wrote: > Hello, > > We have identified a conflict between TICKLESS (CONFIG_NO_HZ) and > the current perfmon2 implementation. The problem impacts system-wide > sessions using timeout-based event set multiplexing. > > Event set multiplexing allows monitoring tools to measure more events > than there are actual performance counters on the processor. Events > are grouped in sets which are then multiplexed onto the actual counters. > Switching can be triggered either by a timeout or by a counter overflow. > This is supported for per-thread and system-wide sessions. > > For timeout-based switching, the duration expressed in nanoseconds is > meant to represent wall-clock time in system-wide mode, and execution > time in per-thread mode. Granularity is limited by HZ. > > The current implementation for timeout is a simple hook on the timer > interrupt path in apic_*.c:smp_local_timer_interrupt(). Unfortunately, > this does not work when tickless is enabled: we get much less set > switches than expected on an idle system. > > It looks like a solution would be to change the implementation of > timeout-based switching to use HR timers instead. Similar to what is > done for ITIMER_REAL and ITIMER_VIRTUAL. > > Unless someone has a better proposal, I will experiment with this on > 2.6.24-rc2. Might help if you CC the tickless folks :-) - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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