linux-kernel - Re: [PATCH] sparse_irq aka dyn

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Message-ID: <491CB421.2020701@sgi.com>
Date:	Thu, 13 Nov 2008 15:11:29 -0800
From:	Mike Travis <travis@....com>
To:	David Miller <davem@...emloft.net>
CC:	paulus@...ba.org, akpm@...ux-foundation.org, yinghai@...nel.org,
	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sparse_irq aka dyn_irq v13

David Miller wrote:
> From: Paul Mackerras <paulus@...ba.org>
> Date: Fri, 14 Nov 2008 09:19:13 +1100
> 
>> Andrew Morton writes:
>>
>>> Other architectures want (or have) sparse interrupts.  Are those guys
>>> paying attention here?
>> On powerpc we have a mapping from virtual irq numbers (in the range 0
>> to NR_IRQS-1) to physical irq numbers (which can be anything) and back
>> again.  I think our approach is simpler than what's being proposed
>> here, though we don't try to keep the irqdescs node-local as this
>> patch seems to (fortunately our big systems aren't so NUMA-ish as to
>> make that necessary).
> 
> This is exactly what sparc64 does as well, same as powerpc, and
> as Paul said it's so much incredibly simpler than the dyn_irq stuff.

One problem is that pre-defining a static NR_IRQ count is almost always
wrong when the NR_CPUS count is large, and should be adjusted as resources
require.

Large UV systems will take a performance hit from off-node accesses
when the CPU count (or more likely the NODE count) reaches some
threshold.  So keeping as much interrupt context close to the
interrupting source is a good thing.
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