netdev - Re: [PATCH net-next] ipv4: PKTINFO doesnt need dst reference

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Date:	Wed, 09 Nov 2011 23:03:03 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	bhutchings@...arflare.com, pstaszewski@...are.pl,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] ipv4: PKTINFO doesnt need dst reference

Le mercredi 09 novembre 2011 à 16:37 -0500, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Wed, 09 Nov 2011 18:24:35 +0100
> 
> > [PATCH net-next] ipv4: IP_PKTINFO doesnt need dst reference
> > 
> > When a socket uses IP_PKTINFO notifications, we currently force a dst
> > reference for each received skb. Reader has to access dst to get needed
> > information (rt_iif & rt_spec_dst) and must release dst reference.
> > 
> > We also forced a dst reference if skb was put in socket backlog, even
> > without IP_PKTINFO handling. This happens under stress/load.
> > 
> > We can instead store the needed information in skb->cb[], so that only
> > softirq handler really access dst, improving cache hit ratios.
> > 
> > This removes two atomic operations per packet, and false sharing as
> > well.
> > 
> > On a benchmark using a mono threaded receiver (doing only recvmsg()
> > calls), I can reach 720.000 pps instead of 570.000 pps.
> > 
> > IP_PKTINFO is typically used by DNS servers, and any multihomed aware
> > UDP application.
> > 
> > Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> 
> Looks good, if it compiles I'll push it out to net-next :-)

Arg :(  I cross my fingers :)

BTW, on my bnx2x adapter, even small UDP frames use more than PAGE_SIZE
bytes :

skb->truesize=4352 len=26 (payload only)

Truesize being now more precise, we hit badly the shared
udp_memory_allocated, even with single frames.

I wonder if we shouldnt increase SK_MEM_QUANTUM a bit to avoid
ping/pong...

-#define SK_MEM_QUANTUM ((int)PAGE_SIZE)
+#define SK_MEM_QUANTUM ((int)PAGE_SIZE * 2)


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