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Round Robin Scheduling Not Power-Efficient
Posted by
kdawson
on Fri May 09, 2008 10:01 AM
from the toward-cooler-server-farms dept.
from the toward-cooler-server-farms dept.
Via_Patrino writes "While having to distribute load between several servers, round robin, or any other technique that balances load equally, is the most common approach because of its simplicity. But a recent study shows that trying to accumulate load on some servers can improve energy efficiency because the other servers will be mostly unused during off-peak periods and then able to make better use of power saving methods. Specially, where load involves lots of concurrent power-consuming TCP connections, which was the case in the study, a new load-balancing algorithm resulted in an overall 30% power savings. Here's the paper (PDF)."
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Logical conclusion (Score:5, Insightful)
Re:Logical conclusion (Score:5, Insightful)
Of course, if you use Amazon EC2, this is all moot, as they can shift load around to have their cluster run at peak efficiency.
Parent
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Re:Logical conclusion (Score:4, Funny)
Parent
Re:Logical conclusion (Score:4)
Get over yourself. Use the right tool for the job.
Parent
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They talk about 30% savings in these applications, but also give 59bn kWh as the figure for total power usage for all data centers, the majority of which probably wouldn't benefit from tweaks suited to persistent connections.
Re: (Score:2, Informative)
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Re:Logical conclusion (Score:5, Interesting)
I'm waiting for the first company to put a data center a few hundred feet under water, where the water temp is low. You'd be surrounded by the worlds biggest heat sink. The environmentalists would have a hissy fit but that's never stopped industry before, and of course you could argue that you are saving electricty on cooling.
Parent
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Screw you, environmentalists!
Re: (Score:2)
Your comment assumes that losing power to the facility is a catastrophe. I say it's not, if it's unmanned.
Yeah, your cooling setup is unpowered, but so are your heat sources.
I bet you could put a set of stilts into the ground, and build a computing environment wrapped around the stilts. Lower the equipment into the water when it's running, and ra
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2: Use some of the electricity to power your data center, and the rest to power other thermodynamically impossible projects.
3: Profit!! (no hidden step needed here, just an impossible one)
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Citations for the idea that the ocean holds heat while the air radiates it harmlessly?
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Re: (Score:2, Interesting)
Apply a connection limit slightly below the performance hockeystick in your load balancers / content switches and you will get maximum power utilization with minimum performance impact.
One other way I see customers getting maxim utilization out of their servers is by using dynamic resource schedule and vmware esx to move virtual servers around behind a load balancer. At
Equity or efficiency? (Score:5, Interesting)
This problem shows up in many places.
Another Scheduling Flamewar (Score:5, Funny)
Re:Another Scheduling Flamewar (Score:5, Funny)
I don't think that we should go down this road again - why don't we talk about religion or politics, instead?
Can we just stick to politics? Why spoil a perfectly good Friday with a vi vs emacs debate?
Parent
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Which candidate should be praying to which diety for good traffic for the moving van to the White House
Managed power supplies... (Score:5, Insightful)
If the load on your boxes is below a threshold, remove one of them from the load balance list, wait for connections to end, or migrate the processes off to another machine, and switch it off. When the load is above a certain threshold, you power on an additional node, configure it for whichever service and add it to the load balancer.
Oh come on people, you call yourselves engineers? It really isn't that difficult.
Re:Managed power supplies... (Score:5, Insightful)
Sure, that's not too difficult to do. But it does add complexity. And it does mean your system can't respond to increased load as quickly, as you have to wait for your additional boxes to boot up. If the increased load is predictable, you can anticipate, but that adds more complexity. It doesn't save you on capital costs as you still have to size your power and A/C systems for peak load. Powering the boxes on and off may shorten their lives or reduce their reliability. The question isn't whether it can be done; it's whether it's worth it.
Parent
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Shutdown Shortens Life? (Score:2)
If you understand complexity in the first place (Score:2)
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it would be silly to have all your boxen running at 5% load because of a dumb load balancing scheme. Energy wasteful to say the least.
LoB
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I am starting to think I didn't miss much by not going to a big name computer science school.
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You'd be surprised how much of engineering is taking "obvious" ideas and banging your head against them for months/years trying to get all the details to work out right.
RR is not load-balancing! (Score:2)
Load balancing is where you actually check the load and then make an informed decision about where to allocate the work.
OK, rant over. Now back to your scheduled programming.
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IT discovers boiler scheduling (Score:5, Interesting)
Operators of multiple steam boilers have been dealing with this problem for a century. The number of boilers fired up is adjusted with demand, with the need for some demand prediction because it takes time to get steam up. This was done manually for decades; now it's often automated.
The same thing applies to multiple HVAC compressors. Usually there's a long-term round-robin switch so that the order of compressor start is rotated on a daily or weekly basis to equalize wear.
More and more, IT is becoming like stationary engineering.
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Re:IT discovers boiler scheduling (Score:4, Insightful)
Which, alas, won't stop someone from patenting it with respect to servers. Even if it's already been done with computers too.
Incidentally, I've seen descriptions of currently available HVAC control systems for office buildings which takes into account the season, the direction the building faces, the thermal mass of the building, demand, etc, and even learns some of these parameters while running, rather than forcing the installer to calculate them. But every office building I've worked in has had crappy systems which amount to running the compressors on a timer and using individually controlled dampers to provide even cooling (poorly). It seems that we have the technology, but not the will (or the capital) to use them.
Parent
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It just seems too obvious for there to not be a solution to this in computing already, let alone it requiring a study to come to this conclusion.
This being so suggests there's more to the story than the summary itself suggests; but to test that I'd have to follow all the li
The real question is what "fully loaded" means (Score:3, Insightful)
The only thing that makes this hard is a metric of what "fully loaded" means for a server. With generators and boilers, you have a single number which represents output, and you know what the capacity of each unit is, so you know when to start up the next unit. Computer servers are more difficult to characterize.
So you have to measure some values of server load, convert that to a single number, and use it for load measurement purposes. Then it all works just like boiler scheduling.
You don't even nee
Pound, haproxy (Score:4, Insightful)
Some kind of loadavg-based, or even response-time, feedback mechanism would be great! Pound has that (I believe), but since Pound requires downtime for every configuration change, we want to move away from it ASAP.
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News, cluster not efficient. (Score:2)
A cluster of computers doing a job is less efficient that a single server doing the same job. Adding to that having a cluster creates more points of failure, and more overhead communicating between those statements.
If you have the option to run the DB & the application on the same server, try to do so.
Very cool, but obvious now that we see it (Score:5, Insightful)
Assuming your system scales smoothly, i.e. gets proportionally slower as the system load starts to exceed processing capacity. For example, a process will always take 100ms as long as there is CPU time to spare, but once the CPU gets to 100% utilization, you have to start time slicing more processes, that 100ms starts to be 150ms. The load balancer can spin up a new server an start bring down the processing times.
This is an obvious solution to an obvious problem, but until now, we've just never had to examine it.
Well, duh... (Score:2, Funny)
BigIP (Score:3, Interesting)
It's new because it's smarter (Score:3, Insightful)
More smarts, I think.
Does your setup allocate ZERO connections to certain servers over some length time, which are set up to reduce energy use upon such zero connections? If not, this looks like it might help.
They're claiming real-world energy efficiency gains, so it looks like it's an improvement somehow.
I would assume it's because this now adds dynamic adjustment, which could be based on total system-stack metrics of peak_load_capability, energy_minimization, acceptable_response_time, etc. Somet
Scientific discoveries 2.0 (Score:2)
Could apply to CPUs as well (Score:3, Insightful)
It would take some targeted software design to take advantage of this, but I think we could be looking at a moores law style increase in power...
Soccer moms and scheduling. (Score:3, Interesting)
A. Spread the kids among all the cars, and drive all the cars (14 cars)
or
B. Fill up a car, and send off. Repeat until done. (6 cars)
What is more energy efficient?
Soccer moms have solved this without statistical analysis or engine torque curves.
Re: (Score:3, Interesting)
Car analogy? Check.
Soccer Moms? Check. Check. (no mention of how many are single though)
But... a lot of soccer moms don't care. They're busy with their other kids and errands too (each server runs more than just apache), so they want the flexibility of driving their own car. Show me a website where the hardware is designed to be energy efficient, and I'll see a site that can't handle a good slashdotting.
Re:Soccer moms and scheduling. (Score:4, Interesting)
First off, you're assuming a standard car with 1 adult driver and 4 passengers; instead, you should be using an SUV with a capacity of 6-8, including driver.
(Result: 4-5 vehicles)
Next, you have to consider that not all parents will attend every game. The primary reason that soccer moms drive SUVs is that they must occasionally transport several of their child's teammates to a game (or, worse, to practice!) when their turn comes up in the rotation. Therefore, you only need enough SUVs to cover the number of child passengers, and the number of adults will follow.
(Result: 2-3 vehicles)
However, you might recall that the other reason that soccer moms drive SUVs is that they often have additional children that have not yet reach sports playing age, and must be transported along with the parent, in a car seat (which, in the case of a standard car, would reduce passenger capacity by at least 20% by rendering the back center seat useless.) Assume that approximately 1 in 3 soccer moms have an additional child to transport, and the child adds to the overall passenger count.
(Result: 3-4 vehicles)
Finally, realizing that the overloaded schedule and priorities of child + parent create scheduling conflicts, it is impossible to get optimal performance. At least 1 child per SUV will be late, leaving a seat empty and requiring another parent with car to tranport them.
(Result: 6-8 vehicles)
The result is a range of possible values, but your initial calculation of 6 vehicles is optimistic at best.
Parent
New Slashdot Headline Needed (Score:3, Informative)
"Round Robin Scheduling Not Power-Efficient when using Windows Live Messenger"
RTFA, in the abstract, "In this paper, we characterize unique properties, performance, and power models of connection servers, based on a real data trace collected from the deployed Windows Live Messenger."
The research itself appears pretty solid. I'd be interested if they publish a followup paper where the model was based off of a variety of applications which utilize round-robin, not just one.
Cooling.current == Server.current (Score:3, Interesting)