device polling and weird timer interrupt count from vmstat

[Artem Kuchin]

Oliver Fromme wrote:
> Artem Kuchin wrote:
>> Oliver Fromme wrote:
>>> Artem Kuchin wrote:
>>>> 3) Is timer int really generated on each cpu? Am i really wasting
>>>> cpu time on ~4000 ints per second?
>>> 
>>> 4000 ints per second is rather nothing on any modern CPU.
>>> Have a look at the top(1) display of an otherwise idle
>>> system.  The "%interrupt" column should be zero, even if
>>> it's processing 4000 timer interrupts per second.  As far
>>> as I know, the cpu timer interrupt handler is very light-
>>> weight.
>> 
>> Thank you for the answer. My convern is that with 4 CPUs i get 8000
>> ints/second.  While em generates only about 200 ints/second. As i
>> undertand not all int handlers are the same. Some are heavy and some
>> are light on CPU. So, the question is what is better (better=less CPU
>> time): 8000 ints/sec from timer or 200 ints/sec from NIC?
> 
> There's no simple answer to the question.  The best way is
> to just try it.  As I wrote, run top(1) while the system is
> idle, so only the cpu timer interrupts are active.  If the
> "%interrupt" column is zero most of the time, then there
> is nothing to worry about at all.
> 
> However, if you have a constant non-zero %interrupt value,
> you might consider lowering HZ, and you might reconsider
> whether polling really has advantages in your situation.
> Do you have reasons to believe so?  Remember that the main
> purpose of polling is to improve interactivity under very
> high network load.  If you're not in such a situation, then
> polling probably doesn't buy you much.

Well, problem with top is that on dual 3GHZ box it alsway s
shows 0% load when not loaded with real traffic (web traffic) no matter
if it is polling of int handling. And when loaded with real traffic
web server eat a lot more cpu power then traffic handling, so, no
separate measurement of traffic cpu load is possible.
But i think it is possible to simulate this kind of load, need to think about
it.

Also, when it comes to public web server i can never be secure enough and
crazy load of traffic can come any time from DDOS attack which can bring
down any box. So, for public web server it is a matter of security and
managebility to have server interactive even on high traffic load. So, even from
this poing of view polling can be usefull.

--
Regards,
Artem