netdev - Re: [PATCH] Software receive packet steering

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Date:	Tue, 21 Apr 2009 11:52:07 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Andi Kleen <andi@...stfloor.org>, netdev@...r.kernel.org,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH] Software receive packet steering

>> Hello Tom
>>
>> I was thinking about your patch (and David's one), and thought it could be
>> possible to spread packets to other cpus only if current one is under stress.
>>
>> A posssible metric would be to test if softirq is handled by ksoftirqd
>> (stress situation) or not.
>>
>> Under moderate load, we could have one active cpu (and fewer cache line
>> transferts), keeping good latencies.
>>
>> I tried alternative approach to solve the Multicast problem raised some time ago,
>> but still have one cpu handling one device. Only wakeups were defered to a
>> workqueue (and possibly another cpu) if running from ksoftirq only.
>> Patch not yet ready for review, but based on a previous patch that was more
>> intrusive (touching kernel/softirq.c)
>>
>> Under stress, your idea permits to use more cpus for a fast NIC and get better
>> throughput. Its more generic.
>
> I would like to see some way to have multiple CPU's pulling packets and adapting
> the number of CPU's being used based on load. Basically, turn all device is into
> receive multiqueue. The mapping could be adjusted by user level (see irqbalancer).
>

That is possible and don't think the design of our patch would
preclude it, but I am worried that each time the mapping from a
connection to a CPU changes this could cause of out of order packets.
I suppose this is similar problem to changing the RSS hash mappings in
a device.

Tom
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