Nice Processes Finish Last

I’ve ran into lots of myths and misconceptions about the Linux “nice” command. Lots of people seem to view it as a magic wand that can be waved at misbehaving processes – they are not sure what it really does but maybe it will make the process nicer?

Indeed there is so much confusion around it, that it is incredibly difficult to find good information about it. Especially since the behavior of nice changes between kernel releases and information that was great two years ago, may not be true today.

Fortunately, Linux is open source and one can always read the source code of the relevant kernel version to know exactly what Nice is doing today. I mean it – the relevant code is all in one file (kernel/sched.c) and is very readable.

Since I’ve already did some reading on the 2.4.31 kernel, I’ll summarize here the behavior of nice in that version and then shoot down specific myths. To explain what Nice does I’ll have to start by a short overview of how Linux Scheduler works. Should be entirely painless and it makes things much clearer later on.

The important part of the scheduler is the schedule() function. This runs every once in a while (I’m not getting into that, except to say that since the scheduler takes CPU, we definitely don’t want it to run too often!), stops the process that is taking CPU now and gives the CPU to a new process. The main question is – which process is going to go next. That’s where the scheduling algorithm comes in.

Scheduling algorithm in Linux works in “epochs” – when the epoch starts, each process in the run queue is allocated certain number of CPU ticks that it can use this epoch. When a process has used all its ticks it needs to give the CPU to another process. If it waits for IO, it doesn’t use any ticks and another process can run at that time, but once it finishes waiting it can use the remaining ticks during the same epoch. When all the processes use all their ticks, the epoch is over and everyone gets more ticks.

So, which process runs next? The algorithm is implemented in goodness() function, which returns a number for each process. The higher the goodness of the process, the more likely it is to run next. Goodness is highest for special real time processes, which always run first. Higher for the process currently using the CPU (because it has things on cache and you don’t want to mess up the cache), high for processes with many ticks left as opposed to those with only few ticks, and 0 for processes that are out of ticks.

Now we get to Nice. Nice impacts the number of ticks a process gets from the CPU. We assign ticks in batches that amount to 50ms, and therefore contain different amounts of ticks for different CPUs, but the basic formula in 2.4 is this:

#define NICE_TO_TICKS(nice)     (TICK_SCALE(20-(nice))+1

TICK_SCALE is where we match the number of ticks to the CPU speed, but as you can see there is an inverse relation between nice value and the number of ticks, where nice 20 gets you the least ticks and -20 gets you the most, and it is a linear relation – when you move from nice 10 to nice 11 it is the same as moving from -5 to -4. This is no longer true in 2.6 where the slope is different for positive and negative values.

To summarize – the nicer you are, the less CPU you get and the less often you will be scheduled. Nice processes will finish last.

What Nice does not do:

  • Many people (including Linux kernel experts!) believe that “nice 20” means your process will only run if no one else wants the CPU. Not exactly. Every process has to use up all their tick counts before the scheduler starts a new epoch (unless the process is sleeping), so while your nice process will probably run last at each epoch, other processes will still wait for it to finish before the new epoch starts and they get a new tick quota.
  • Nice will not have much effect on a low utilization system. Your CPU is at 0% now. You start a heavy process (maybe export) with nice 20, what will happen? It will take its usual 70% CPU. Because as long as there are no queues this nice process will be rescheduled again and again. What’s the rest of the CPU doing? running the scheduler, which is now working much harder since it has to reschedule this process again and again with a tiny quota each time.
  • Top shows 100% cpu utilization, and you see that 80% of it is taken by a single process. Should you nice it? Not always. Top will show IOWait time as part of the CPU time, but processes don’t use ticks while they wait for IO, so if the process is IO bound, it will show a very similar behavior regardless of nice. The behavior is to use very little CPU, wait long time for IO, and show on Top as a very busy process.
  • Export takes tons of CPU and I don’t want other processes to wait for it, I don’t mind if export takes a bit longer. Should I nice? Probably not. For one thing, export is IO bound, which makes it insensitive to nice anyway. What is more important – export doesn’t do most of the work. What is doing the work is Oracle’s server process that is opened to work with export, and you actually have to “renice” the oracle child process if you hope for anything at all to happen. What you can nice with pretty good effect it the “zip” process that you run after the export.
  • And if we mentioned the child processes, keep in mind that when a process forks the child process gets the same “nice” value as the parent, but from that point on you can renice each of them independently.

3 Comments on “Nice Processes Finish Last”

  1. Freek says:

    Good post.
    It made things a lot more clear to me.

  2. Daniel Zollinger says:

    That was very nicely explained. Thank you for the post.

  3. […] 9, 2008 A while ago I explained Nice, what it can do and what it can’t. From this point, whenever someone misunderstood nice, I […]

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