netdev - Re: [RFC v2: Patch 1/3] net: hand off skb list to other cpu to submit to upper layer

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Message-Id: <20090313.115137.254924980.davem@davemloft.net>
Date:	Fri, 13 Mar 2009 11:51:37 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	therbert@...gle.com
Cc:	yanmin_zhang@...ux.intel.com, bhutchings@...arflare.com,
	andi@...stfloor.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, herbert@...dor.apana.org.au,
	jesse.brandeburg@...el.com, shemminger@...tta.com
Subject: Re: [RFC v2: Patch 1/3] net: hand off skb list to other cpu to
 submit to upper layer

From: Tom Herbert <therbert@...gle.com>
Date: Fri, 13 Mar 2009 10:06:56 -0700

> You'll definitely want to look at the hardware provided hash.  We've
> been using a 10G NIC which provides a Toeplitz hash (the one defined
> by Microsoft) and a software RSS-like capability to move packets from
> an interrupting CPU to another for processing.  The hash could be used
> to index to a set of CPUs, but we also use the hash as a connection
> identifier to key into a lookup table to steer packets to the CPU
> where the application is running based on the running CPU of the last
> recvmsg.  Using the device provided hash in this manner is a HUGE win,
> as opposed to taking cache misses to get 4-tuple from packet itself to
> compute a hash.  I posted some patches a while back on our work if
> you're interested.

I never understood this.

If you don't let the APIC move the interrupt around, the individual
MSI-X interrupts will steer packets to individual specific CPUS and as
a result the scheduler will migrate tasks over to those cpus since the
wakeup events keep occuring there.
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