linux-kernel - Re: PATCH: Allow over-ride of smp_found

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Date:	Fri, 06 Feb 2009 22:00:54 -0500 (EST)
From:	Len Brown <lenb@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Ben Greear <greearb@...delatech.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: PATCH:  Allow over-ride of smp_found_cfg with kernel cmd-line
 option.

> > Without this override, performance drops by around 15% on
> > network throughput tests on this system.
> 
> The question is, _why_ does network performance drop?
>
> Do you have a before/after pair of /proc/interrupts output
> perhaps? Does the layout of IRQs change?
>
> The system is not SMP - so only something about UP-IOAPIC can
> be helpful to performance here - or some unknown effect.

I presume it is IOAPIC (good) vs PIC (bad), for the 
patch that provoked this regression was one that caused
the MPS table to be disabled.  (the problem with that patch
was that it was effective not just in ACPI mode, but in acpi=off mode)

The other un-answered question is why the system under test
has ACPI disabled in the BIOS when it did not ship that
way from the manufacturer.

cheers,
-Len
Intel Open Source Technology Center
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