linux-kernel - CONFIG_IRQBALANCE for 64-bit x86 ?

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Date:	Mon, 19 Nov 2007 23:12:21 -0500
From:	Mark Lord <lkml@....ca>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...l.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: CONFIG_IRQBALANCE for 64-bit x86 ?

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 ?
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