netdev - Re: rps perfomance WAS(Re: rps: question

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Date:	Sat, 17 Apr 2010 01:43:39 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	hadi@...erus.ca, Changli Gao <xiaosuo@...il.com>,
	Rick Jones <rick.jones2@...com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	robert@...julf.net, andi@...stfloor.org
Subject: Re: rps perfomance WAS(Re: rps: question

> So the cost of queing the packet into our own queue (netif_receive_skb
> -> enqueue_to_backlog) is about 0.74 us  (74 ms / 100000)
>
> I personally think we should process packet instead of queeing it, but
> Tom disagree with me.
>
You could do that, but then the packet processing becomes HOL blocking
on all the packets that are being sent to other queues for
processing-- remember the IPIs is only sent at the end of the NAPI.
So unless the upper stack processing is <0.74us in your case, I think
processing packets directly on the local queue would improve best case
latency, but would increase average latency and even more likely worse
case latency on loads with multiple flows.

> RPS on, directed on cpu1 (other socket)
> (echo 02 > /sys/class/net/eth3/queues/rx-0/rps_cpus)
> 100000 packets transmitted, 100000 received, 0% packet loss, time 4542ms
>
> So extra cost to enqueue to a remote cpu queue, IPI, softirq handling...
> is 3 us. Note this cost is in case we receive a single packet.
>
> I suspect IPI itself is in the 1.5 us range, not very far from the
> queing to ourself case.
>
> For me RPS use cases are :
>
> 1) Value added apps handling lot of TCP data, where the costs of cache
> misses in tcp stack easily justify to spend 3 us to gain much more.
>
> 2) Network appliance, where a single cpu is filled 100% to handle one
> device hardware and software/RPS interrupts, delegating all higher level
> works to a pool of cpus.
>
> I'll try to do these tests on a Nehalem target.
>
>
>
>
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