linux-kernel - Re: [PATCH 0/8] git pull request for tip/tracing/core

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Date:	Wed, 11 Feb 2009 14:39:55 -0600
From:	Jack Steiner <steiner@....com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Mike Travis <travis@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"isimatu.yasuaki@...fujitsu.com" <isimatu.yasuaki@...fujitsu.com>,
	"kaneshige.kenji@...fujitsu.com" <kaneshige.kenji@...fujitsu.com>
Subject: Re: [PATCH 0/8] git pull request for tip/tracing/core

On Wed, Feb 11, 2009 at 09:57:00AM -0800, Luck, Tony wrote:
> > > Before we go and make the change, Peter brought up a good point on IRC. Is
> > > there any reason that ia64 needs 1 << 14 IRQs?  That's 16384!
> > >
> > > Perhaps the better solution wolud be (if possible), to simply lower the
> > > number of bits.
> >
> > i'm the wrong person to be asked about that. (Cc:-ed the right people)
> 
> People build some pretty big systems on ia64.  SGI's largest has 4096
> cpus ... so 16384 IRQs is only 4 per cpu.  That doesn't sound like very
> many to me.

As long as per-cpu interrupts (ex., timer, cpe_hndlr, cmc_hndlr, etc) are not counted
in the IRQ count, SGI is ok with  256 IRQs. We had problems overflowing the old
limit of 256 but have worked around it.

If per-cpu interrupts ARE counted in the total IRQs, then we currently use 5
per-cpu interrupts for each cpu. A 4k cpu config will need 20k IRQs + additional
IRQs for real devices.

I know the IRQ infrastructure has changed recently so this may no longer apply.


--- jack
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