[MacTUG] Macs, energy consumption and the 3 Stooges...

[Donald Duff-McCracken]

Hi there

As you can imagine, we in the Faculty of Environment are trying to reduce the energy consumption of our Macs, as should anyone! I have been having a problem which I thought was a technical issue, but it has lead me to some interesting conclusions.

I have the macs set up to go to sleep after 60 minutes, the display to sleep after 20 mins. This has been totally fine and led to know issues. However, to save a bit more energy, I had also scheduled the macs to shutdown in the evening, and start up in the morning. While a mac does not consume a lot of energy when it is sleeping (most of them are now less than 2W), when you have whole classrooms of them it can add up to a modest amount.

Only recently, did I clue into the fact that most of the macs were not shutting down at night. A little trolling on the web made me realize that currently a mac cannot shutdown if it is sleeping — which makes sense as the mac is only barely alive when it is sleeping.

Now I realize that using pmset I could probably resolve this by doing something like scheduling a wake of up the machine up so that it could shut down — which reminds me of the old 3 Stooges 'wake up and go to sleep' routine (http://www.youtube.com/watch?v=fVywCYLIUz4). But as I thought a bit more about this I realized that this would not save me any energy, and 'letting sleeping Macs lie' was saving more energy.

Here is the scenario I wanted. The machine shuts down at 11pm and restarts at 7am. I will presume that by 11 pm, most computers are probably sleeping. (This is confirmed by the fact that very few of my macs were shutting down at 11 as they were in a deep sleep.) Therefore the energy I am saving is ~16 Wh (compared to sleeping for 8 hours @2 watts).

However, I was having my macs restart every day at 7am. I was doing this as I did not want labs of students walking in and having to restart the computers in the morning (but waking them from sleep is fine). The problem is that the macs are then awake for an hour before fully falling asleep. During this hour they will consume 42 Wh* -- in other words, if shutting them down is only practical if I have them automatically restart, then I save much more energy by just letting them sleep!

As to letting them stay sleeping for long periods of time, I really do not see an issue with this, I can always wake them with ARD for any updates or whatever I want to happen.

So for those of you wanting to shut down your macs to save energy, run your own calculations. It may not be saving any energy to do anything other than sleep mode. A Mac does not have to be running for very long to eat up the equivalent of the energy consumed by hours of sleep mode.

Now if there are any of you out there that are not sleeping your Macs, you should take a long hard look as to why not. Is there really anything happening in the night when you need to have the Macs be awake? You can always wake them with ARD, or if it is really important you can schedule a wake. We have students running renders that can take hours on our Macs, and the energy saving software is smart enough to not cause the Macs to fall asleep part way though a render. At the very least you should set your screens to dim after a short period of time (which saves 2/3s of the total energy) and also have the Macs go to bed when most users in the labs have gone home.


*The 42Wh is based on: 30Wh for the 20 minutes they are fully awake (90watts*.33hrs), and ~12Wh when the monitor is dimmed but they are in idle mode (37.5watts*.66hrs). In reality the energy used would likely be more as it looks like I would need to wake up the Macs before shutting them down. So there would be a period of time (5-10 min?) where the Mac would be chugging at 90W before it was shut down (even 5 min or fully awake time is 7.5Wh which is equal to almost 4 hours of sleep mode), so really I would be looking at something like 50Wh per machine if I wanted them to shut down to save energy!!




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Donald Duff-McCracken
Technical Services Manager
Mapping, Analysis & Design
Faculty of Environment
University of Waterloo
(519) 888-4567 x32151
http://www.environment.uwaterloo.ca/computing/people/don.html
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