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

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Date:	Mon, 6 Jul 2009 14:58:33 -0700
From:	Chetan.Loke@...lex.Com
To:	<daniel.blueman@...il.com>, <matthew@....cx>, <andi@...stfloor.org>
CC:	<linux-kernel@...r.kernel.org>, <jens.axboe@...cle.com>,
	<arjan@...radead.org>
Subject: RE: >10% performance degradation since 2.6.18

> -----Original Message-----
> From: linux-kernel-owner@...r.kernel.org 
> [mailto:linux-kernel-owner@...r.kernel.org] On Behalf Of 
> Daniel J Blueman
> Sent: Sunday, July 05, 2009 7:01 AM
> To: Matthew Wilcox; Andi Kleen
> Cc: Linux Kernel; Jens Axboe; Arjan van de Ven
> 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.
> 

Possible enhancement - 

1) Drain the responses in the xmit_frame() path. That is, post the TX-request() and just before returning see if there are
   any more responses in the RX-queue. This will minimize(only if the NIC f/w coalesces) interrupt load.
   The n/w core should drain the responses rather than calling the drain-routine from the adapter's xmit_frame() handler. This way there won't be any need to
   modify individual xmit_frame handlers.


PS - I'm not familiar with the networking code.



Chetan Loke--
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