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. Thanks. -- -Stephane - 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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