netdev - Re: [PATCH net-next 2/2] udp: implement and use per cpu rx skbs cache

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Date:   Wed, 18 Apr 2018 09:56:22 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next 2/2] udp: implement and use per cpu rx skbs cache



On 04/18/2018 03:22 AM, Paolo Abeni wrote:
> This changeset extends the idea behind commit c8c8b127091b ("udp:
> under rx pressure, try to condense skbs"), trading more BH cpu
> time and memory bandwidth to decrease the load on the user space
> receiver.
> 
> At boot time we allocate a limited amount of skbs with small
> data buffer, storing them in per cpu arrays. Such skbs are never
> freed.
> 
> At run time, under rx pressure, the BH tries to copy the current
> skb contents into the cache - if the current cache skb is available,
> and the ingress skb is small enough and without any head states.
> 
> When using the cache skb, the ingress skb is dropped by the BH
> - while still hot on cache - and the cache skb is inserted into
> the rx queue, after increasing its usage count. Also, the cache
> array index is moved to the next entry.
> 
> The receive side is unmodified: in udp_rcvmsg() the usage skb
> usage count is decreased and the skb is _not_ freed - since the
> cache keeps usage > 0. Since skb->usage is hot in the cache of the
> receiver at consume time - the receiver has just read skb->data,
> which lies in the same cacheline - the whole skb_consume_udp() becomes
> really cheap.
> 
> UDP receive performances under flood improve as follow:
> 
> NR RX queues	Kpps	Kpps	Delta (%)
> 		Before	After
> 
> 1		2252	2305	2
> 2		2151	2569	19
> 4		2033	2396	17
> 8		1969	2329	18
> 
> Overall performances of knotd DNS server under real traffic flood
> improves as follow:
> 
> 		Kpps	Kpps	Delta (%)
> 		Before	After
> 
> 		3777	3981	5


It might be time for knotd DNS server to finally use SO_REUSEPORT instead of
adding this bloat to the kernel ?

Sorry, 5% improvement while you easily can get 300% improvement with no kernel change
is not appealing to me :/