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Message-ID: <47F34625.6000600@jp.fujitsu.com> Date: Wed, 02 Apr 2008 17:39:01 +0900 From: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com> To: Paul Jackson <pj@....com> CC: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>, Andi Kleen <andi@...stfloor.org> Subject: Re: [PATCH 1/2] Customize sched domain via cpuset Paul Jackson wrote: > Interesting ... Thank you for saying that ;-) > So, we have two flags here. One flag "sched_wake_idle_far" that will > cause the current task to search farther for an idle CPU when it wakes > up another task that needs a CPU on which to run, and the other flag > "sched_balance_newidle_far" that will cause a soon-to-idle CPU to search > farther for a task it might pull over and run, instead of going idle. > > I am tempted to ask if we should not elaborate this in one dimension, > and simplify it in another dimension. > > First the simplification side: do we need both flags? Yes, they are > two distinct cases in the code, but perhaps practical uses will always > end up setting both flags the same way. If that's the case, then we > are just burdening the user of these flags with understanding a detail > that didn't matter to them: did a waking task or an idle CPU provoke > the search? Do you have or know of a situation where you actually > desire to enable one flag while disabling the other? Yes, we need both flags. At least in case of hackbench (results are attached bottom), I couldn't find any positive effect with enabling "sched_wake_idle_far", but "sched_balance_newidle_far" shows significant gains. It doesn't mean "sched_wake_idle_far" is useless everywhere. As Peter pointed, when we have a lot of very short running tasks, "sched_wake_idle_far" accelerates task propagation and its throughput. There are definitely such situations (and in fact it's where I'm now). Put simply, if the system tend to be idle, then "push to idle" strategy works well. OTOH if the system tend to be busy, then "pull by idle" strategy works well. Else, both strategy will work but besides of all there is a question: how much searching cost can you pay? So, it is case by case, depend on the situation. > For the elaboration side: your proposal has just two-level's of > distance, near and far. Perhaps, as architectures become more > elaborate and hierarchies deeper, we would want N-level's of distance, > and the ability to request such load balancing for all levels "n" > for our choice of "n" <= N. > > If we did both the above, then we might have a single per-cpuset file > that took an integer value ... this "n". If (n == 0), that might mean > no such balancing at all. If (n == 1), that might mean just the > nearest balancing, for example, to the hyperthread within the same core, > on some current Intel architectures. If (n == 2), then that might mean, > on the same architectures, that balancing could occur across cores > within the same package. If (n == 3) then that might mean, again on > that architecture, that balancing could occur across packages on the > same node board. As architectures evolve over time, the exact details > of what each value of "n" mean would evolve, but always higher "n" > would enable balancing across a wider portion of the system. > > Please understand I am just brain storming here. I don't know that > the alternatives I considered above are preferrable or not to what > your patch presents. Now we already have such levels in sched domain, so if "n" is given, I can choice: 0: (none) 1: cpu_domain - balance to hyperthreads in a core 2: core_domain - balance to cores in a package 3: phys_domain - balance to packages in a node ( 4: node_domain - balance to nodes in a chunk of nodes ) ( 5: allnodes_domain - global balance ) It looks easy... but how do you handle if cpusets are overlapping? Thanks, H.Seto ----- (@ CPUx8 ((Dual-Core Itanium2 x 2 sockets) x 2 nodes), 8GB mem) [root@...KBENCH]# echo 0 > /dev/cpuset/sched_balance_newidle_far [root@...KBENCH]# echo 0 > /dev/cpuset/sched_wake_idle_far [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.956 Time: 4.008 Time: 5.918 Time: 8.269 Time: 10.216 [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.918 Time: 3.964 Time: 5.732 Time: 8.013 Time: 10.028 [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.925 Time: 3.824 Time: 5.893 Time: 7.975 Time: 10.373 [root@...KBENCH]# echo 0 > /dev/cpuset/sched_balance_newidle_far [root@...KBENCH]# echo 1 > /dev/cpuset/sched_wake_idle_far [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 2.153 Time: 3.749 Time: 5.846 Time: 8.088 Time: 9.996 [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.845 Time: 3.932 Time: 6.137 Time: 8.062 Time: 10.282 [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.963 Time: 4.040 Time: 5.837 Time: 8.017 Time: 9.718 [root@...KBENCH]# echo 1 > /dev/cpuset/sched_balance_newidle_far [root@...KBENCH]# echo 0 > /dev/cpuset/sched_wake_idle_far [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.725 Time: 3.412 Time: 5.275 Time: 7.441 Time: 8.974 [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.674 Time: 3.334 Time: 5.374 Time: 7.204 Time: 8.903 [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.689 Time: 3.281 Time: 5.002 Time: 7.245 Time: 9.039 [root@...KBENCH]# echo 1 > /dev/cpuset/sched_balance_newidle_far [root@...KBENCH]# echo 1 > /dev/cpuset/sched_wake_idle_far [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.923 Time: 3.697 Time: 5.632 Time: 7.379 Time: 9.223 [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.809 Time: 3.656 Time: 5.746 Time: 7.386 Time: 9.399 [root@...KBENCH]# for i in `seq 50 50 250` ; do ./hackbench $i ; done Time: 1.832 Time: 3.743 Time: 5.580 Time: 7.477 Time: 9.163 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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