On Fri, Apr 12, 2013 at 11:38:04PM -0700, Paul E. McKenney wrote: > On Fri, Apr 12, 2013 at 04:54:02PM -0700, Josh Triplett wrote: > > On Fri, Apr 12, 2013 at 04:19:13PM -0700, Paul E. McKenney wrote: > > > From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> > > > > > > Systems with HZ=100 can have slow bootup times due to the default > > > three-jiffy delays between quiescent-state forcing attempts. This > > > commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based > > > on the value of HZ. However, this would break very large systems that > > > require more time between quiescent-state forcing attempts. This > > > commit therefore also ups the default delay by one jiffy for each > > > 256 CPUs that might be on the system (based off of nr_cpu_ids at > > > runtime, -not- NR_CPUS at build time). > > > > > > Reported-by: Paul Mackerras <pau...@au1.ibm.com> > > > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > > > > Something seems very wrong if RCU regularly hits the fqs code during > > boot; feels like there's some more straightforward solution we're > > missing. What causes these CPUs to fall under RCU's scrutiny during > > boot yet not actually hit the RCU codepaths naturally? > > The problem is that they are running HZ=100, so that RCU will often > take 30-60 milliseconds per grace period. At that point, you only > need 16-30 grace periods to chew up a full second, so it is not all > that hard to eat up the additional 8-12 seconds of boot time that > they were seeing. IIRC, UP boot was costing them 4 seconds.
I added some instrumentation, which counted 202 calls to synchronize_sched() during boot (Fedora 17 minimal install + development tools) with a 3.8.0 kernel on a 4-cpu KVM virtual machine on a POWER7. Without this patch, those 202 calls take up a total of 4.32 seconds; with it, they take up 3.6 seconds. The kernel is compiled with HZ=100 and NR_CPUS=1024, like the standard Fedora kernel. I suspect a lot of the calls are in udevd and related processes. Interestingly there were no calls to synchronize_rcu_bh or synchronize_sched_expedited. Paul. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
