Re: [PATCH 0/8] use jump labels to streamline common APIC configuration

[Avi Kivity <avi@redhat.com>]

On 08/05/2012 10:30 PM, Eric Northup wrote:
> On Sun, Aug 5, 2012 at 5:58 AM, Gleb Natapov <gleb@redhat.com> wrote:
>> APIC code has a lot of checks for apic presence and apic HW/SW enable
>> state.  Most common configuration is when each vcpu has in kernel apic
>> and it is fully enabled. This path series uses jump labels to turn checks
>> to nops in the common case.
> 
> What is the target workload and how does the performance compare?  As
> a naive question, how different is it than just using gcc branch
> hints?

We saw about 1.3% spent in kvm_apic_present() in some OLTP benchmark.

I imagine most of it would be gone by just inlining.  An exception would
be the call from kvm_irq_delivery_to_apic() which happens from a
different cpu, possibly causing a cache miss if something in the same
cache line as vcpu->arch.apic is changed frequently.

Come to think of it, if we implemented the irq routing cache (which
eliminates the loop in kvm_irq_delivery_to_apic()) we might improve
things quite a lot as well.

-- 
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