Messages in this thread | | | Date | Tue, 22 Sep 2009 20:31:44 +0200 | From | Arjan van de Ven <> | Subject | Re: Intel Local APIC behavior on low frequencies |
| |
On Tue, 22 Sep 2009 12:31:24 -0400 pradeep hettiarachchi <pacprnt@gmail.com> wrote:
> Hi, > > Here’s a description of my problem. It would be great if you can find > some info on this matter. > > > > I need to find some information about local APIC and CPU frequency > scaling behavior. Currently I am modifying the Linux kernel for my > research, which involves studying a custom job scheduler I am > developing at different CPU frequencies. As we know, if the kernel > works in tick-less mode (and in hi resolution mode) it uses the local > APIC as a clock event device and as the clock tick device. > > > > My working environment is: Linux vanilla kernel 2.6.24, Intel core 2 > Quad Q6600, Intel core 2 duo T6500 processors. Gcc compiler and the > tool chain. > > The information I am trying to find is: > > 1) I want to find-out the tick count(say for a 100 ms period) of the > local APIC when the CPU operates at different operating frequencies. > When I study this matter, I found that the local APIC does not tick > (does not work accurately) reliably as the clock frequency is varied. > It works accurately in the maximum frequency of the system. Do you > know any reason for that? or it is the way it operates?(is that > normal?) > > (my observation: when I count the LAPIC tick count for the maximum CPU > frequency, it matches with the number of ticks that the system > generates at the time of the initial LAPIC calibration, which happens > at the very beginning of the kernel initialization; however when I try > to count the number of ticks with scaled frequencies, the LAPIC > "current counter" difference is very less, some times less than 100 > for 100 ms, which should not be true.) > > I don't have good reason to think that, some other task re-programs > the LAPIC: I set the LAPIC "initial counter" to a huge value, further > I assume when the system works in hi resolution mode, the re-program > happens from the LAPIC handler "hrtimer_interrupt" which I replace > with a dummy handler. > > Thank you in advance.
the local apic timer is not impacted by the cpu frequency. HOWEVER, it will stop when the CPU is idle. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |