Re: About IO-APIC - General question

[Klaas Zweck <khz@tzi.org>]

hi peter,

i made some test on a linux-smp machine(kernel 2.4.9).
i had one process just doing some calculation on prim numbers.
i locked this to cpu 1 and disabled all interrupts on cpu 1.
( in fact i routed all to cpu 0 )
then i started a make -j4 bzImage job in a remote
shell to produce file i/o and network interrupts.
i restricted all processes except the one doing the calculation
from cpu 1 and delivered all interrupts only to cpu 0.

i had an total runtime for the calculating process of:
(with interrupts routed only to one cpu)

voyager:~/# time ./rt_prim_ohne_syscalls
real    0m42.111s
user    0m41.870s
sys     0m0.240s

voyager:~/# time ./rt_prim_ohne_syscalls
 
real    0m40.268s
user    0m40.190s
sys     0m0.080s
voyager:~/# time ./rt_prim_ohne_syscalls
 
real    0m40.068s
user    0m40.000s
sys     0m0.070s
 
average :
real:   0m40.815s 

in the same scenario as above but with irqs routed to all cpus
the calculationg process still locked to cpu 1 had a runtime of:


 
voyager:~/# time ./rt_prim_ohne_syscalls -rt
 
real    0m39.712s
user    0m39.680s
sys     0m0.030s
voyager:~/# time ./rt_prim_ohne_syscalls -rt
 
real    0m39.998s
user    0m39.720s
sys     0m0.270s
voyager:~/# time ./rt_prim_ohne_syscalls -rt
 
real    0m39.950s
user    0m39.900s
sys     0m0.050s
 
 
average:
real    0m39.886s


so its a measurable but small performance plus of 2.5 %.
but since i worked remote just network and i/o irqs occeured there were
no keyboard or mouse irqs in the system.


hope it helps a bit,
greetings,
klaas

Peter H. Koenig wrote:

>Hello,
>
>from how I understood APIC it purpose on an SMP machine is to give all
>processors the possibility to handle interrupts instead of one processor
>responsible for them. So when running two jobs on an SMP machine a
>non-working IO-APIC would turn into a performance disadvantage. Do I get
>this right ? If yes, does anybody have a clue, how much difference this
>could make when running IO-intensive programms ? single or two digit
>percentages ?
>
>Thanks
>Pete
>
>
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