netdev - Re: Bad network performance over 2Gbps

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Date:	Tue, 15 Apr 2008 13:34:59 -0700
From:	"Kok, Auke" <auke-jan.h.kok@...el.com>
To:	"H. Willstrand" <h.willstrand@...il.com>
CC:	Anton Titov <a.titov@...t.bg>, netdev@...r.kernel.org,
	Jesse Brandeburg <jesse.brandeburg@...el.com>
Subject: Re: Bad network performance over 2Gbps

H. Willstrand wrote:
> [Changed mail list]
> 
> On Tue, Apr 15, 2008 at 8:06 PM, Anton Titov <a.titov@...t.bg> wrote:
>> I use Linux for serving a huge amount of static web on few servers. When
>>  network traffic goes above 2Gbit/sec ksoftirqd/5 (not every time 5, but
>>  every time just one) starts using exactly 100% CPU time and packet
>>  packet loss starts preventing traffic from going up. When the network
>>  traffic is lower than 1.9Gbit ksoftirqds use 0% CPU according to top.
>>
>>  Uplink is 6 gigabit Intel cards bonded together using 802.3ad algorithm
>>  with xmit_hash_policy set to layer3+4. On the other side is Cisco 2960
>>  switch. Machine is with two quad core Intel Xeons @2.33GHz.
>>
>>  Here goes a screen snapshot of "top" command. The described behavior
>>  have nothing to do with 13% io-wait. It happens even if it is 0%
>>  io-wait.
>>  http://www.titov.net/misc/top-snap.png
>>
>>  kernel configuration:
>>  http://www.titov.net/misc/config.gz
>>
>>  /proc/interrupts, lspci, dmesg (nothing intresting there), ifconfig,
>>  uname -a:
>>  http://www.titov.net/misc/misc.txt.gz
>>
>>  Is it a Linux bug or some hardware limitation?

I'm wondering if this is not a classical demonstration of the NAPI-irq trap where
after migration all the interrupts from the various cards are migrated to a single
CPU, and because of NAPI once they're busy polling won't ever migrate away from
that CPU again.

Have you looked at `cat /proc/interrupts` before and after this happens?

My guess is that your specific situation can benefit from setting smp_affinity and
forcing the NIC irq's so that you're at least occupying the load over multiple
CPU's (but preferably ones that use the same cache!) will help relieve the situation.

alternatively you might even see an improvement by disabling NAPI. depending on
the driver that you're using this might be possible.

I actually don't know much about bonding and how this affects everything, but my
guess is that that's a less important factor in this issue.

Cheers,

Auke
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