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Round Robin Scheduling Not Power-Efficient

Posted by kdawson on 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)."

11 of 141 comments (clear)

  1. Logical conclusion by DoofusOfDeath · · Score: 4, Insightful

    So if we're willing to sacrifice speed for energy savings, shouldn't we just use the bare minimum number of computers that can handle the workload without crashing?

    1. Re:Logical conclusion by TooMuchToDo · · Score: 4, Insightful
      What this means is someone needs to architect an intelligent loading system. Ideally, it would manage the load on your base load servers (that are on all the time), and when those servers reach 85-95% of capacity (numbers from my ass) other servers should be brought out of low power/sleep mode to start serving.

      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.

  2. Managed power supplies... by Colin+Smith · · Score: 4, Insightful

    Just switch them off...

    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.

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    1. Re:Managed power supplies... by russotto · · Score: 4, 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.


      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.
    2. Re:Managed power supplies... by PenguiN42 · · Score: 2, Insightful

      Oh come on people, you call yourselves engineers? It really isn't that difficult.

      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.

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      The following sentence is true. The preceding sentence was false.
  3. Pound, haproxy by QuoteMstr · · Score: 3, Insightful

    We're running a no-frills OpenBSD load balancer at work. Right now, it's running Pound (the quickest thing we could get up once traffic spiked a few weeks ago), but we're considering other approaches too. haproxy's load balancing knobs look interesting. It looks like you can configure it so the maximum number of clients scales with the current load. The problem is that there's no feedback system.

    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.

  4. Very cool, but obvious now that we see it by mlwmohawk · · Score: 4, Insightful

    This is a very cool idea, and I don't think it will affect usability too much either. As long as the load balancer keeps tabs on system loading, via snmp or something, it can turn on/off machines based on need.

    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.

  5. Re:IT discovers boiler scheduling by russotto · · Score: 3, Insightful

    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.


    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.
  6. The real question is what "fully loaded" means by Animats · · Score: 2, 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 need to do much advance planning, as you have to do with boilers and generators, since you can usually start up another server in a minute or so. It takes hours to fire up a big boiler, so you need serious prediction capability.

    (The classic power company approach was a chart recorder recording system load. Every day, somebody took the day's load graph and cut out a piece of cardboard to match. The cardboard pieces were accumulated in a rack, and the result was a 3D load graph for the year. It looks like a mountain range. There's an Internet Archive film showing this. That's a worthwhile exercise for your server farm, and you can probably do it without glue and scissors today. I've seen some of Amazon's server load graphs, which have a huge peak entering the Xmas buying season. In fact, the real reason Amazon is selling "cloud computing" is that their plant is sized for the holiday season and 80% idle the rest of the year.)

  7. Could apply to CPUs as well by bill_kress · · Score: 2, Insightful

    I think it's probably simplistic to simply distribute a load to all cores of a CPU evenly. Although asymmetrical might be tougher, I could see a system with one low-power always-on core to deal with system requests and organization (Maybe even low enough power to remain on during a suspend), One to handle all GUI threads and interact with the GPU on a private bus, a couple normal cores to handle typical user threading, one of which doesn't come on until the first is like 50% loaded, and one or two high-speed high-power cores that run all-out when the system is plugged in and needs them for intensive processing.

    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...

  8. It's new because it's smarter by Steve+Hamlin · · Score: 2, 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. Something that seems to be lacking in the current load-balancing system that you describe.

    Future:
    - Allow for tuning based on diverse hardware, each with different energy and load capability profiles.
    - Smart managing of a large population of these systems, based on varying load.
    - Add real-time upstream energy cost data into the mix
    - Dynamic scheduling of administrative tasks based on energy efficiency vs. hard deadlines.
    - If energy starts becoming a significant cost of hosting, go back to selling system time based, in part, on total energy used - track CPU, disk, network energy requirements by watt/hr, by user. Add those to account plans, side-by-side with Mb/s and GB/month. Serving .cn? Host your site out of a data center in Iceland and take advantage of cheap, midnight power!