netdev - Re: [PATCH RFC net-next] netif_receive

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Message-ID: <1430284980.3711.38.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 28 Apr 2015 22:23:00 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Thomas Graf <tgraf@...g.ch>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH RFC net-next] netif_receive_skb performance

On Tue, 2015-04-28 at 19:11 -0700, Alexei Starovoitov wrote:
> Hi,
> 
> there were many requests for performance numbers in the past, but not
> everyone has access to 10/40G nics and we need a common way to talk
> about RX path performance without overhead of driver RX. That's
> especially important when making changes to netif_receive_skb.

Well, in real life, having to fetch RX descriptor and packet headers are
the main cost, and skb->users == 1.

So its nice trying to optimize netif_receive_skb(), but make sure you
have something that can really exercise same code flows/stalls,
otherwise you'll be tempted by wrong optimizations.

I would for example use a ring buffer, so that each skb you provide to
netif_receive_skb() has cold cache lines (at least skb->head if you want
to mimic build_skb() or napi_get_frags()/napi_reuse_skb() behavior)

Also, this model of flooding one cpu (no irqs, no context switch) mask
latencies caused by code size, since icache is fully populated, with a
very specialized working set.

If we want to pursue this model (like user space (DPDK and alike
frameworks)), we might have to design a very different model than the
IRQ driven one, by dedicating one or multiple cpu threads to run
networking code with no state transition.

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