Messages in this thread | | | Date | Tue, 4 May 2010 22:14:00 +0200 | From | Dominik Brodowski <> | Subject | Re: [RFC PATCH v2] nohz/sched: disable ilb on !mc_capable() |
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On Tue, May 04, 2010 at 03:14:24PM +0200, Peter Zijlstra wrote: > On Mon, 2010-04-26 at 22:31 +0200, Dominik Brodowski wrote: > > From: Dominik Brodowski <linux@dominikbrodowski.net> > > Date: Thu, 8 Apr 2010 21:51:18 +0200 > > Subject: [PATCH] nohz/sched: disable ilb on !mc_capable() > > > > On my dual-core, !mc_capbale() CPU, the idle load balancer (ilb) is one > > of the main reasons ticks are not stopped: Under moderate load (~98 % idle), > > upt o half of the calls to tick_nohz_top_sched_tick() are aborted due > > to calls to select_nohz_load_balancer(1). > > > > I suspect this is caused by the following phenomenon: > > > > CPU0 CPU1 > > <active> <active> > > tick_nohz_stop_sched_tick(1) > > select_nohz_load_balancer(1) > > => CPU0 becomes ilb owner, <CPU1 becomes idle a bit later> > > tick is not stopped, tick_nohz_stop_sched_tick(1) > > CPU0 goes to sleep for => CPU1 isn't the ilb owner, > > exactly 1 tick. tick is stopped. > > <short sleep> <long sleep> > > ---> scheduler_tick() > > tick_nohz_stop_sched_tick(0) > > tick_nohz_stop_sched_tick(1) > > => is ilb owner, all CPUs are > > idle, CPU0 may go to sleep. > > > > If all CPU cores have hardly anything to do, letting the active CPU do > > idle load balancing allows us to enter deep sleep states earlier, and for > > longer periods of time. Furthermore, on !mc_capable() systems, it seems that > > the ilb algorithm isn't needed at all. Let's show this for a 2-core system: > > > > - if both cores are active, ilb is deactivated > > - if no core is active, ilb is deactivated > > - if only one core is active, it attempts to balance its load off to other > > CPUs on each tick anyway. ilb wouldn't act quicker. > > > > This patch decreases the amount of wakeups on my completely idle notebook by > > about two thirds. > > Right, so I think the !mc_capable() check is buggy, at the very least on > sparc64 which is 'creative' with its sched_domain maps. > > I'm also not sure what a single socket AMD Magny-Cours will do. > > On a single socket Nehalem we will have a non trivial sched_domain > because we also have the threads included. > > I think we can only do your optimization for machines that end up having > a single sched_domain that covers the entire machine.
Is there an easy way to determine there's just a single sched_domain?
Best, Dominik
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