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Ulrich Drepper

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Energy saving is everybody's business [Nov. 8th, 2007|08:41 pm]
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With the wide acceptance of laptop and even smaller devices more and more people have been exposed to devices limited by energy consumption. Still, programmers don't pay much attention to this aspect.

This statement is not entirely accurate: there has been a big push towards energy conservation in the kernel world (at least in the Linux kernel). With the tickless kernels we have the infrastructure to sleep for long times (long is a relative term here). Other internal changes avoid unnecessary wakeups. It is now realy up to the userlevel world to do its part.

The situation is pretty dire here. There are some projects (e.g., PowerTOP) which highlight the problems. Still, not much happens.

I've been somewhat guilty myself. nscd (part of glibc) was waking up every 5 seconds to clean up its cache, even if often was to be done. This program structure has several reasons. Good ones, but not ultimate reason. So I finally bit the bullet and changed the program structure significantly to better enable wakeup. The result is that now nscd at all times determines when the next cache cleanup is due and sleeps until then. Cache cleanups might be many hours out, so the code improved from one wakeups every 5 seconds to one wakeup every couple of hours.

nscd is a very small drop in the bucket, though. Just look at your machine and examine the running processes and those which are regularly started. PowerTOP cannot realy help here (Arjan said something will be coming soon though).

There is a tool which can help, though: systemtap. Simply create a small script which traps syscalls the violators will use and disply process information. The syscalls to use include: open, stat, access, poll, epoll, select, nanosleep, futex. For the latter five it is a matter of small timeout values which is the problem.

I'll post a script to do this soon (just not now). But the guilty parties probably already know who they are. Just don't do this quasi busy waiting!

  • If a program has to react to a file change or removal or creation, use inotify
  • for internal cleanups, choose reasonable values and then compute the timeout so that you don't wake up when nothing has to be done.

If you want to see how not to do it, look at something like the flash player (the proprietary one). If you inadvertently have started it it'll remain active (even if no flash page is displayed) and it is basically busy waiting on something.

Let's show the proprietary software world we can do better.

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