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Rethinking power-aware scheduling

Rethinking power-aware scheduling

Posted Jan 13, 2012 23:23 UTC (Fri) by dlang (subscriber, #313)
In reply to: Rethinking power-aware scheduling by mjg59
Parent article: Rethinking power-aware scheduling

It's exactly what people are used to.

on their laptops, when they unplug the screen dims slightly and the systems switch to a more aggressive power saving mode.

your customers are not asking for it explicitly because they are used to getting it by default.

most of them won't realise what the problem is if they don't get it, they will just consider their device sluggish (or at least not as fast as the competition) if they don't get peak performance when plugged in, and they will consider the device/OS to be a power hog if it doesn't last as long when on battery power.


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Rethinking power-aware scheduling

Posted Jan 13, 2012 23:28 UTC (Fri) by mjg59 (subscriber, #23239) [Link]

Since we mostly sell into the enterprise server market, I'm pretty sure that that's not what they're talking about.

Rethinking power-aware scheduling

Posted Jan 13, 2012 23:39 UTC (Fri) by dlang (subscriber, #313) [Link]

Ok, I don't know who you are or what you market, but I also don't know very many enterprise servers that have battery powered modes.

Rethinking power-aware scheduling

Posted Jan 13, 2012 23:44 UTC (Fri) by mjg59 (subscriber, #23239) [Link]

None. That's the point. They want aggressive power management despite these devices always being plugged in. The assumption that just because you're not running off battery you're not interested in power management is one that's untrue for a huge proportion of Linux users. It's in no way an accurate heuristic.

Rethinking power-aware scheduling

Posted Jan 14, 2012 0:02 UTC (Sat) by dlang (subscriber, #313) [Link]

so if you want aggressive power management, you set it. nobody is preventing it.

But setting aggressive power management for all cases as the default for everyone is wrong.

Rethinking power-aware scheduling

Posted Jan 14, 2012 22:43 UTC (Sat) by raven667 (subscriber, #5198) [Link]

But powering more hardware than required to run the workload is wasteful of power for no benefit. Isn't it a good idea to work on the scheduler so that it can run the computer just as hard with just as much power use as necessary and no more? You may want a tunable to make the power saving so aggressive that it affects performance, although the wisdom IIRC is that making workloads run slow makes them use more power by running longer. Making the default no power saving at all is probably not reasonable.

Rethinking power-aware scheduling

Posted Jan 14, 2012 22:48 UTC (Sat) by dlang (subscriber, #313) [Link]

there is no way to have power savings with no performance penalty under any conditions.

it takes time to bring CPUs out of sleep states, and during that time the work that is waiting for them may not be able to get done.

it is not always less power to run at full speed and then sleep, that is frequently the case, but it depends on the ability to move in and out of sleep, along with the amount of power saved.

In this case, we are talking about the options when you have multiple cores, some sharing components, and have less work than it takes to max out all the cores.

putting all the work on one core and powering off the other cores may save power, but it could make the work take longer (but not enough longer to use more power than the other cores would consume if they were not powered down). for some people having the work take slightly longer won't matter, for others it will.

Rethinking power-aware scheduling

Posted Jan 15, 2012 3:53 UTC (Sun) by raven667 (subscriber, #5198) [Link]

There is no reason to think that a power aware scheduler can't be good enough to be the default, is there? Even for latency sensitive operations the scheduler could keep some amount of idle capacity available for bursts of work without running the whole machine at full bore. It seems to me that power saving should be the default even for machines on mains power

Rethinking power-aware scheduling

Posted Jan 15, 2012 13:48 UTC (Sun) by mjg59 (subscriber, #23239) [Link]

Nonsense. Turbo mode is an example of aggressive power management resulting in significantly enhanced performance under certain workloads.

Rethinking power-aware scheduling

Posted Jan 15, 2012 11:09 UTC (Sun) by liljencrantz (guest, #28458) [Link]

What makes you so sure that your intuition on what is an accurate heuristic for power management is so much better than Matthew Garret's? As a kernel developer that seems to work almost full time on power issues for Red Hat, one would hope that he has a more than passing familiarity with the needs of the enterprise market. If your answer is along the lines of intuition/personal experience, then perhaps you should consider the possibility that your needs are the atypical ones? If your answer is something entirely different, then please elucidate us, because right now it might seem like you're stating opinions as facts.

My somewhat limited personal experience is that most data centers I've worked with have a total power limit per rack that is painfully low, and that reducing power usage by a few watts per system would allow us to stuff in one more server per rack, leading to a significant amount of savings. This resonates well with what Garret is saying.


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