Re: about interrupt latency

[zyphr <infzyphr@gmail.com>]

On Sat, 12 Mar 2005 10:01:43 -0700 (MST), Zwane Mwaikambo
<zwane@arm.linux.org.uk> wrote:
> On Wed, 9 Mar 2005, Francesco Oppedisano wrote:
> 
> > On Tue, 8 Mar 2005 12:09:58 -0700 (MST), Zwane Mwaikambo
> > <zwane@arm.linux.org.uk> wrote:
> >
> > > At some cpu frequency point on i386 the main cause of your interrupt
> > > service latency will be in the interrupt controller and how long from irq
> > > assertion to the signal being recognised, resultant vector being
> > > dispatched to the processor and the necessary interrupt controller
> > > acknowledge steps required. This is also helped by the fact that the
> > > Linux/i386 interrupt vector stubs are very small and fast, so there isn't
> > > all that much code to execute to reach the ISR from the vector table. I'm
> > > not sure if you've tested this, but you may notice that timer interrupt
> > > via Local APIC will have lower dispatch latency than timer interrupt via
> > > i8259 only. But that's all at the lower end of the latency graph, you will
> > > most likely run into other sources on a busy system.
> > >
> > >         Zwane
> > >
> > Very interesting zwane....i haven't tested the local APIC....do you
> > think this dispatch time can vary with the system I/O load (many
> > pending interrupts in the PIC)?
> 
> The PIC/i386 setup cannot really pend interrupts so i would say no i don't
> think the dispatch time would be affected.
> 
> > I think the interrupt latency is influenced even by the code inside
> > the kernel: if a lot of code is running with interrupts disabled then
> > the interrupt latency will grow. Am i right?
> 
> Yes that's correct, in fact this will be your major/main cause of
> interrupt latency.
> 
> > So probably we can state that the factors influencing the interrupt latency are:
> > 1)Dispatching time in the PIC
> > 2)Waiting time on the PIC (if there are pending interrupt of lower vector)
> 
> With APIC/i386 this is more possible, if you're really interested you
> should try and calculate the dispatch time using the same system (must
> have an IOAPIC) and testing with the following combinations;
> 
> PIC only (noapic, nolapic)
> PIC + LAPIC (noapic)
> IOAPIC + LAPIC
> 
> You will probably find that the IOAPIC + LAPIC has the lowest dispatch
> time. Also worth noting that the LAPIC can queue upto 2 vectors (on P4),
> this allows for more interrupts to be ready for dispatch.
> 
> > 3)fetching ISR from main memory
> > 4)wait time when CPU is running with disabled interrupt
> >
> > Do U agree?
> 
> A bit more on (4)
> 
> 4a) wait time when CPU is running firmware (e.g. SMI/SMM, VGA BIOSes etc)
> 
> Otherwise, yes i agree, good luck with your research.
> 
>         Zwane
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I am just a user, not sure if it is related.
But with CONFIG_X86_UP_APIC and/or  CONFIG_X86_UP_IOAPIC set my mouse
behaviour is totally different, when playing games everything feels
more "laggy".
And it gives me what gamers call "negative acceleration".