Re: [gentoo-server] vmstat cs and in

[Jeroen Geilman]

Richard Broersma Jr wrote:
> 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.

J

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