Many mfgpt timer interrupts with tickless kernel

[Jens Axboe]

On Thu, Mar 22 2007, William Cohen wrote:
> Hi all,
> 
> I was trying to make sense of the data that I collected earlier with the 
> idle1.stp probe. It displayed the percentage of time spent in halted mode, 
> which the highest I saw on an idle machine was 17 percentage. This seemed 
> rather low for a machine that was doing nothing, so I took a closer look at 
> what was going on the idle machine. All this work is based on kernel built 
> from dilinger's:
> 
> kernel-2.6.21-20070312.1.olpc.3eca75102a57502.src.rpm
> 
> I ran an experiment where the machine state idle for 10 seconds with the 
> systemtap script running. I also compared the number of interrupts before 
> and after the experiment. There is about 150 interrupts per second from the 
> mfgpt timer. I thought with the tickless kernel that this would be a lot 
> lower.
> Below is the data from the experiment. I tried booting the machine in 
> runlevel three and got the same results, about 150 interrupts per second. 
> Why so many timer interrupts for tickless kernel?

The tickless kernel just _enables_ low number of interrupts per second,
it'll still wakeup everytime something is scheduled to run. So you
probably want to look into _why_ th timer interrupts happen so often -
who is the user of them?

I see ~70 interrupts/sec here with an idle machine in X, FWIW.

-- 
Jens Axboe