On 11/11/09 10:44, Tim Blechmann wrote: > i am curious, do the values of CONFIG_HZ and CONFIG_NO_HZ somehow affect > the scheduling latency (as reported by cyclictest) of a > real-time/vanilla kernel, especially when CONFIG_HIGH_RES_TIMERS is enabled? > > thnx, tim > Some anecdotal info... From: http://lkml.indiana.edu/hypermail/linux/kernel/0906.2/03853.html "I have disabled CONFIG_NOHZ long ago, since it really influences latency." >From http://www.mail-archive.com/linux-rt-users@xxxxxxxxxxxxxxx/msg02300.html "With CONFIG_NO_HZ=y or CONFIG_PPC_BESTCOMM_GEN_BD=y the latency increases by approx. 100..150us, each." The thread goes on for a while, but I don't think the CONFIG_NO_HZ question was resolved. Looking at some old source (2.6.23.17-rt14), there was at least one longer IRQ disabled path if CONFIG_NO_HZ: void __noinstrument irq_enter(void) { #ifdef CONFIG_NO_HZ int cpu = smp_processor_id(); if (idle_cpu(cpu) && !in_interrupt()) tick_nohz_stop_idle(cpu); #endif ... #ifdef CONFIG_NO_HZ if (idle_cpu(smp_processor_id())) tick_nohz_update_jiffies(); #endif } But this code is different in later kernels. As of 2.6.29.4-rt19, it was: irq_enter() ... if (idle_cpu(cpu) && !in_interrupt()) { __irq_enter(); tick_check_idle(cpu); // tick_check_idle() contains: #ifdef CONFIG_NO_HZ tick_nohz_stop_idle(cpu); tick_nohz_update_jiffies(); tick_nohz_kick_tick(cpu); - Frank Rowand Sony Corporation of America -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
