netdev - Re: [net-next PATCH 3/5] pktgen: avoid atomic

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Date:	Wed, 14 May 2014 07:35:31 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	netdev@...r.kernel.org,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	Florian Westphal <fw@...len.de>,
	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <shemminger@...tta.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Robert Olsson <robert@...julf.se>,
	Ben Greear <greearb@...delatech.com>,
	John Fastabend <john.r.fastabend@...el.com>, danieltt@....se,
	zhouzhouyi@...il.com
Subject: Re: [net-next PATCH 3/5] pktgen: avoid atomic_inc per packet in
 xmit loop

On Wed, 2014-05-14 at 16:17 +0200, Jesper Dangaard Brouer wrote:
> Avoid the expensive atomic refcnt increase in the pktgen xmit loop, by
> simply setting the refcnt only when a new SKB gets allocated. Setting
> it according to how many times we are spinning the same SKB (and
> handling the case of skb_clone=0).
> 
> Performance data with CLONE_SKB==100000 and TX ring buffer size=1024:
>  (single CPU performance, ixgbe 10Gbit/s, E5-2630)
>  * Before: 5,362,722 pps --> 186.47ns per pkt (1/5362722*10^9)
>  * Now:    5,608,781 pps --> 178.29ns per pkt (1/5608781*10^9)
>  * Diff:    +246,059 pps -->  -8.18ns
> 
> The performance increase converted to nanoseconds (8.18ns), correspond
> well to the measured overhead of LOCK prefixed assembler instructions
> on my E5-2630 CPU which is measured to be 8.23ns.
> 
> Note, with TX ring size 768 I see some "tx_restart_queue" events.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---

OK, but then you need to properly undo the refcnt at the end of pktgen
loop, otherwise you leak one skb ?

Say you run pktgen for 995 sends, and clone_count is 10.

If done properly, you probably can avoid the kfree_skb(pkt_dev->skb) in
pktgen_xmit(), and set initial skb->users to max(1, pkt_dev->clone_skb);





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