[SDBUG] interrupts in top broken with SMP kernel

[Mike Joyce]

Hey Mike,

When I am benching systems for hardware performance there are two main
utilities I use.

1. vmstat(8)
The area in vmstat you are interested are the two columns concerning interrupts:
   System
       in: The number of interrupts per second, including the clock.
       cs: The number of context switches per second.

You will also be interested in the 'id' column, if this drops into
single digits you might have interrupt misses or other odd problems.
Especially if you are working in a realtime environment, like a DNS
server, Firewall, or VoIP application.

2. top(1)

Top has many bits of useful information. However there is the 'hi'
indicator at the top. I always assumed that this was 'hardware
interrupts', however with a quick look through the man page I couldn't
find any description of that column. I have seen a direct corelation
between the hi column and activity on the pci bus. IE: If I add more
cards, the 'hi' column will increase.


One important thing to note, is that when I am adding cards to the PCI
bus I see a non linear increase in hardware interrupt load. Below are
a few examples:

1 pci card: 2% hi load
2 pci cards: 15% hi load
3 pci cards: 60% hi load

I have attributed this behavior with both cards requesting interrupts
at the same time, and having to retry, thus queuing up interrupts over
time.

Hope that helps.

Cheers!
Mike

On 4/9/07, Michael J McCafferty <mike at m5computersecurity.com> wrote:
> All,
>     When I use the generic uniproc kernel and run top or systat vmstat
> I can get info on the % of cpu for interrupts. However, when using the
> generic bsd.mp kernel on a Core2Duo system I can not get anything to
> tell me the % of cpu for interrupts.
>     I can get the number of interrupts per second, but not the % of
> the CPU that is handlign interrupts. This is a metric I need to know
> as it will help me extrapolate the ultimate performance limit of the
> systems I am testing.
>     Does anyone have any thoughts on how to determine the % of CPU
> that is handlign intgerrupts on a Core2Duo with teh mp kernel ?
>
> Thanks,
> Mike
>
> --
> ************************************************************
> Michael J. McCafferty
> Principal, Security Engineer
> M5 Hosting
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-- 
Mike Joyce
mjoyce at obstinate.org