linux-kernel - Re: [PATCH] i386 IOAPIC: de-fang IRQ compression

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Date:	Wed, 05 Dec 2007 16:25:11 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Natalie Protasevich" <protasnb@...il.com>
Cc:	"Len Brown" <lenb@...nel.org>, "Andi Kleen" <ak@...e.de>,
	"Ingo Molnar" <mingo@...hat.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386 IOAPIC: de-fang IRQ compression

"Natalie Protasevich" <protasnb@...il.com> writes:

> On Nov 27, 2007 10:21 PM, Len Brown <lenb@...nel.org> wrote:
>>     commit c434b7a6aedfe428ad17cd61b21b125a7b7a29ce
>>     (x86: avoid wasting IRQs for PCI devices)
>>     created a concept of "IRQ compression" on i386
>>     to conserve IRQ numbers on systems with many
>>     sparsely populated IO APICs.
>>
>>     The same scheme was also added to x86_64,
>>     but later removed when x86_64 recieved an IRQ over-haul
>>     that made it unnecessary -- including per-CPU
>>     IRQ vectors that greatly increased the IRQ capacity
>>     on the machine.
>>
>>     i386 has not received the analogous over-haul,
>>     and thus a previous attempt to delete IRQ compression
>>     from i386 was rejected on the theory that there may
>>     exist machines that actually need it.  The fact is
>>     that the author of IRQ compression patch was unable
>>     to confirm the actual existence of such a system.
>
> Those systems did exist (and still exist actually). They used over 200
> irqs sometimes and with "normal" IRQ allocation they were failing even
> before reaching half of their I/O configuration. So simple removal
> wouldn't work for those, dynamic allocation sure would. They "scrolled
> off the topic" though because new generations of such machines are not
> 32 bit anymore. So the author didn't actually object :) it was the
> other users of large 32 bit platforms that did.

Natalie.  Did they just have over 200 irqs/gsis or did they actually
use over 200 irqs?

In particular is a large NR_IRQS plus dynamic vector allocation
sufficient for all cases you know about?

Eric
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