linux-kernel - Re: >10% performance degradation since 2.6.18

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Date:	Sun, 5 Jul 2009 12:00:31 +0100
From:	Daniel J Blueman <daniel.blueman@...il.com>
To:	Matthew Wilcox <matthew@....cx>, Andi Kleen <andi@...stfloor.org>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: >10% performance degradation since 2.6.18

On Jul 3, 9:10 pm, Arjan van de Ven <ar...@...radead.org> wrote:
> On Fri, 3 Jul 2009 21:54:58 +0200
>
> Andi Kleen <a...@...stfloor.org> wrote:
> > > That would seem to be a fruitful avenue of
> > > investigation -- whether limiting the cards to a single RX/TX
> > > interrupt would be advantageous, or whether spreading the eight
> > > interrupts out over the CPUs would be advantageous.
>
> > The kernel should really do the per cpu binding of MSIs by default.
>
> ... so that you can't do power management on a per socket basis?
> hardly a good idea.
>
> just need to use a new enough irqbalance and it will spread out the
> interrupts unless your load is low enough to go into low power mode.

I was finding newer kernels (>~2.6.24) would set the Redirection Hint
bit in the MSI address vector, allowing the processors to deliver the
interrupt to the lowest interrupt priority (eg idle, no powersave)
core (http://www.intel.com/Assets/PDF/manual/253668.pdf pp10-66) and
older irqbalance daemons would periodically naively rewrite the
bitmask of cores, delivering the interrupt to a static one.

Thus, it may be worth checking if disabling any older irqbalance
daemon gives any win.

Perhaps there is value in writing different subsets of cores to the
MSI address vector core bitmask (with the redirection hint enabled)
for different I/O queues on heavy interrupt sources? By default, it's
all cores.

Daniel
-- 
Daniel J Blueman
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