Re: [gentoo-server] vmstat cs and in

[Richard Broersma Jr]

> > according to man vmstat:
> >
> >    System
> >        in: The number of interrupts per second, including the clock.
> >        cs: The number of context switches per second.
> >
> > 1) Does anyone have a "layman's" explaination of what interrupts and context switches are?
> >
> Interrupts are just that - signals that interrupt the CPU in whatever it
> is doing and command it to do something else that needs to be done right
> away.
> There are two varieties: hardware and software.
> The hardware type is invoked by an actual hardware signal on the CPU -
> the system timer is one of these, but so is the keyboard :)
> The software interrupts can be programmed by anyone, and are often used
> as a handy shortcut to run a common task without a lot of overhead.
>
> A context switch is the CPU switching from one execution context to
> another; task switches are context switches, but so are kernel system
> calls from userspace programs.
> Since the kernel runs in a different execution environment than all the
> other processes, every switch from one to the other needs to save and
> later restore the context of that process - stack space, registers,
> flags, etc.
>
> So the two are sort of related: every interrupt that causes a kernel
> function to be called (like the process scheduler), also causes a
> context switch when it suspends it current task and resumes the next task.
>
> > 2) How high can these numbers be before a system admin should start to worry about
> preformance?
> >
> Erm... they don't have much to do with performance, they just happen; if
> they didn't, you wouldn't have a working system...
> Just FYI: the 2.6 kernel uses a 1000Hz timer for the process scheduler,
> so expect to see at least 1000 interrupts per second no matter what else
> your system is doing.
> If the number drops much *lower* than this, you might have a performance
> problem: the CPU can't keep up with your tasks.
> Same goes for context switches: the busier it is, the more switches will
> have to take place, except this need not happen on every interrupt -
> only when a task actually needs to be executed.
> > 3) what are the options to reduce these number?
> >
> Stop using your computer.
> Seriously :)
>
> If you really want to know what is going on , I suggest you start using
> _dstat_ instead of vmstat - it offers more information and will also
> help to correlate between the different numbers, e.g. an increase in
> disk or network activity will also show an increase in interrupts, and a
> maxed-out disk subsystem will show up as a higher system CPU percentage
> than normal.
>
> But apart from the pretty nifty info you can get from dstat, don't worry
> about the values; it's just not that important.
> The disk and cpu usage are much more informative in that respect.

Thanks!
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