linux-kernel - Re: [PATCH] Re: Bad network performance over 2Gbps

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Message-ID: <20080421131937.GA4685@ucw.cz>
Date:	Mon, 21 Apr 2008 15:19:38 +0200
From:	Pavel Machek <pavel@....cz>
To:	"Kok, Auke" <auke-jan.h.kok@...el.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Anton Titov <a.titov@...t.bg>, Chris Snook <csnook@...hat.com>,
	"H. Willstrand" <h.willstrand@...il.com>, netdev@...r.kernel.org,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Re: Bad network performance over 2Gbps

Hi!

> [X86] IRQBALANCE: Mark as BROKEN and disable by default
> 
> The IRQBALANCE option causes interrupts to bounce all around on SMP systems
> quickly burying the CPU in migration cost and cache misses. Mainly affected are
> network interrupts and this results in one CPU pegged in softirqd completely.
> 
> Disable this option and provide documentation to a better solution (userspace
> irqbalance daemon does overall the best job to begin with and only manual setting
> of smp_affinity will beat it).
> 
> Signed-off-by: Auke Kok <auke-jan.h.kok@...el.com>
> 
> ---
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 6c70fed..956aa22 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1026,13 +1026,17 @@ config EFI
>    	platforms.
> 
>  config IRQBALANCE
> -	def_bool y
> +	def_bool n

ACK.
>  	prompt "Enable kernel irq balancing"
> -	depends on X86_32 && SMP && X86_IO_APIC
> +	depends on X86_32 && SMP && X86_IO_APIC && BROKEN

This is wrong. irqbalance works, there's nothing wrong with it; but it
has nasty sideffects.

>  	help
>  	  The default yes will allow the kernel to do irq load balancing.
>  	  Saying no will keep the kernel from doing irq load balancing.
> 
> +	  This option is known to cause performance issues on SMP
> +	  systems. The preferred method is to use the userspace
> +	  'irqbalance' daemon instead. See http://irqbalance.org/.
> +

ACK.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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