linux-kernel - Re: CONFIG_IRQBALANCE for 64-bit x86 ?

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Message-ID: <20071120042929.GD15227@1wt.eu>
Date:	Tue, 20 Nov 2007 05:29:29 +0100
From:	Willy Tarreau <w@....eu>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Mark Lord <lkml@....ca>, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...l.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: CONFIG_IRQBALANCE for 64-bit x86 ?

On Tue, Nov 20, 2007 at 03:17:15PM +1100, Nick Piggin wrote:
> On Tuesday 20 November 2007 15:12, Mark Lord wrote:
> > On 32-bit x86, we have CONFIG_IRQBALANCE available,
> > but not on 64-bit x86.  Why not?
> >
> > I ask, because this feature seems almost essential to obtaining
> > reasonable latencies during heavy I/O with fast devices.
> >
> > My 32-bit Core2Duo MythTV box drops audio frames without it,
> > but works perfectly *with* IRQBALANCE.
> >
> > My QuadCore box works very well in 32-bit mode with IRQBALANCE,
> > but responsiveness sucks bigtime when run in 64-bit mode (no IRQBALANCE)
> > during periods of multiple heavy I/O streams (USB flash drives).
> >
> > That's with both the 32 and 64 bit versions of Kubuntu Gutsy,
> > so the software uses pretty much identical versions either way.
> >
> > As near as I can tell, when IRQBALANCE is not configured,
> > all I/O device interrupts go to CPU#0.
> >
> > I don't think our CPU scheduler takes that into account when assigning
> > tasks to CPUs, so anything sent to CPU0 runs with very high latencies.
> >
> > Or something like that.
> >
> > Why no IRQ_BALANCE in 64-bit mode ?
> 
> For that matter, I'd like to know why it has been decided that the
> best place for IRQ balancing is in userspace. It should be in kernel
> IMO, and it would probably allow better power saving, performance,
> fairness, etc. if it were to be integrated with the task balancer as
> well.

Agreed. When userspace has something to do with the way IRQs are
delivered, it's going to smell as bad as micro-kernels...

Willy

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