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

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Date:	Sun, 20 Apr 2008 15:08:43 +0300
From:	Denys Fedoryshchenko <denys@...p.net.lb>
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>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Re: Bad network performance over 2Gbps

By default also without IRQBALANCE enabled in kernel, APIC or someone else distributing interrupts over processors too.
There is no irqbalance daemon or whatever.

For example:
Router-KARAM ~ # cat /proc/interrupts
           CPU0       CPU1
  0:   87956938 1403052485   IO-APIC-edge      timer
  1:          0          2   IO-APIC-edge      i8042
  9:          0          0   IO-APIC-fasteoi   acpi
 19:        140       5714   IO-APIC-fasteoi   ohci_hcd:usb1, ohci_hcd:usb2
 24:  675673280 1186506694   IO-APIC-fasteoi   eth2
 26:  717865662 2201633562   IO-APIC-fasteoi   eth0
 27:    1869190   23075556   IO-APIC-fasteoi   eth1
NMI:          0          0   Non-maskable interrupts
LOC: 1403052485   87956683   Local timer interrupts
RES:      75059      25408   Rescheduling interrupts
CAL:      99542         83   function call interrupts
TLB:        616        200   TLB shootdowns
TRM:          0          0   Thermal event interrupts
SPU:          0          0   Spurious interrupts
ERR:          0
MIS:          0

sunfire-1 ~ # cat config|grep -i irq
CONFIG_GENERIC_HARDIRQS=y
CONFIG_GENERIC_IRQ_PROBE=y
CONFIG_GENERIC_PENDING_IRQ=y
# CONFIG_IRQBALANCE is not set
CONFIG_HT_IRQ=y
# CONFIG_HPET_RTC_IRQ is not set
CONFIG_TRACE_IRQFLAGS_SUPPORT=y
# CONFIG_DEBUG_SHIRQ is not set

Is it harmful too?

On Thursday 17 April 2008 20:37, Kok, Auke wrote:
> Anton Titov wrote:
> > On Tue, 2008-04-15 at 16:59 -0400, Chris Snook wrote:
> >> Still, I think you're on to something here.  Disabling NAPI and instead 
> >> tuning the cards' interrupt coalescing settings might allow irqbalance 
> >> to do a better job than it is currently.
> > 
> > Disabling NAPI allowed me to push as much as 3.5Gbit out of the same
> > server with ~ 20% of time CPUs doing software interrupts.
> 
> yes, I really don't see this is such an amazing discovery - the in-kernel
> irqbalance code is totally wrong for network interrupts (and probably for most
> interrupts).
> 
> on your system with 6 network interrupts it blows chunks and it's not NAPI that is
> the issue - NAPI will work just fine on it's own. By disabling NAPI and reverting
> to the in-driver irq moderation code you've effectively put the in-kernel
> irqbalance code to the sideline and this is what makes it work again.
> 
> It's not the right solution.
> 
> We keep seing this exact issue pop up everywhere - especially with e1000(e)
> datacenter users - this code _has_ to go or be fixed. Since there is a perfectly
> viable solution, I strongly suggest disabling it.
> 
> This is not the first time I've sent this patch out in some form...
> 
> Auke
> 
> 
> ---
> [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
>  	prompt "Enable kernel irq balancing"
> -	depends on X86_32 && SMP && X86_IO_APIC
> +	depends on X86_32 && SMP && X86_IO_APIC && BROKEN
>  	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/.
> +
>  config SECCOMP
>  	def_bool y
>  	prompt "Enable seccomp to safely compute untrusted bytecode"
> --
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Virtual ISP S.A.L.
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