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List:       linux-smp
Subject:    Re: Multiple APIC performance improvement ?
From:       James Bottomley <James.Bottomley () columbiasc ! ncr ! com>
Date:       1999-05-28 15:17:57
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RH132926@exchange.ColumbiaSC.NCR.com said:
> I had a look at the patches to 2.3.2 to support multiple APICs.
> Has anybody seen any noteworthy performance changes after implementing
> them ?

Well, yes otherwise I wouldn't have written them.  The performance improvement 
comes in two forms

1) If your machine cannot be set up in single IO-APIC mode, linux will 
automatically boot in XT-PIC mode which is a major performance drag in an SMP 
system.

2) Even if you don't have a problem with 1) multiple IO-APICs are used to lift 
the degeneracy of the PCI interrupts and prevent device IRQ sharing.  Exactly 
the type of speed up you see from this depends on what devices you have 
sharing an interrupt.  In a fully loaded server class machine this can be a 
major win given the time it takes to find out which card wants the interrupt.  
If you don't share interrupts in single IO-APIC mode there will be no 
improvement.

James Bottomley


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