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Date: Thu, 03 Apr 2008 12:21:09 +0900 From: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com> To: Paul Jackson <pj@....com> CC: linux-kernel@...r.kernel.org, mingo@...e.hu, peterz@...radead.org, andi@...stfloor.org Subject: Re: [PATCH 1/2] Customize sched domain via cpuset Paul Jackson wrote: > Hidetoshi wrote: >> 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 each flag has value in some cases ... that much seems reasonable to me. > > But you're saying that you'd like to avoid having to turn on both, just to > get the benefit of one of them, in order to avoid the searching costs of > the other flag that was not valuable on that load, right? > > But is this necessarily so? I'd like to turn on both(since I know it is best for my application/system), but it can't be denied that there are other situations loving only one of them... At least there is a small possible conflict: "Are you idle?" - "No, I'm busy to search a busy CPU!" To be honest, I don't have strong reason to have them to be divided. Just I thought that they could work independently and it might be usable interface for other people. (... well, I would be a little happy if I don't need to rewrite almost all of the additional piece of Documentation/cpuset.txt, but don't care :-D) So, if there is no one can find use of two flags, I'll change it to one. Comments from any others? > If "pull by idle" is attempted on a system > which tends to be idle, then while it is true that the search for something > to pull will usually find nothing, what does it matter that we wasted some > otherwise idle cycles, looking for pullable, runnable tasks that cannot be > found, on a system that is mostly idle? > > If "push to idle" is attempted on a system that is quite busy, then > couldn't that be coded to notice rather quickly if any nearby CPUs are > idle, and not search if there are no idle neighbors. One could imagine > a word of memory for each smaller domain ("neighborhood") of CPUs (say > all the logical CPUs in a package), with one bit per logical CPU, that > was set if-and-only-if that CPU was in idle. Then it would be very > quick for all the CPUs in that domain to see if there are (or just > were ... close enough) any idle CPUs, and skip trying to "push to idle" > if that word was all zero bits. That is, there would be no sense > trying to push to idle if there were no idle CPUs to push to. The only > writing and the only locking of that word would be from idle loop code, > and only from nearby CPUs in the same small domain, so it would not be > an impediment to large system scaling or a waste of many CPU cycles on > busy systems. > > With a little work such as this, we could make it so that anytime you > needed either flag, you could turn on both, and the other one would be > harmless enough ... just a minor consumer of otherwise idle cycles. > > Then with that, we could have one flag, that did both. I believe there are quite technical reasons why we have no "idle_map." Excellent answers would be brought by scheduler folks... >> It looks easy... but how do you handle if cpusets are overlapping? > > Yeah - that part might be challenging. Would it work to always take > the largest domain balancing requested? Hum... if one requests "smaller" and another is "don't care = default", we always take "default" range. Anyway, I'd like to give a lot of care to well-defined cpusets, and I know that balancing on overlapping cpusets are easy to be confused, so I'll update my patch to take levels, getting in your suggestion. Thanks, H.Seto -- 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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