Subject: Re: [RFC] skb align patch

From: Eric Dumazet <[email-masked]>
Date: Tue, 22 Sep 2009 05:20:53 +0200

Stephen Hemminger a écrit :
> On Mon, 21 Sep 2009 08:13:20 +0200
> Eric Dumazet <[email-masked]> wrote:
> 
>> Stephen Hemminger a écrit :
>>> Based on the Intel suggestion that PCI-express overhead is
>>> a significant cost.
>>>
>>> Would people doing performance please measure the impact of
>>> changing SKB alignment (64 bit only).
>> I had this idea some time ago when I hit a limit on bnx2 adapter
>> (Giga bit link, BCM5708S), with small packets. pktgen was able
>> to send ~500 Mbps 'only', or 700kps if I remember well.
>> So I tried to align the pktgen build packet to a cache line,
>> it gave no difference at all, but it was on a 32 bit kernel.
>> (Thus my patch was for pktgen only, not a generic one as yours)
>>
>> Could you elaborate why this change could be useful on 64bit ?
>>
> 
> It is useful on all architecture where unaligned CPU access is
> relatively cheap.
> 
> The issue is that a unaligned DMA requires a read/modify/write
> cache line access versus just a write access. I am not a bus
> expert, but writes are probably more pipelined as well.
> 

Oh I see, you want to optimize the rx (NIC has to do a DMA
to write packet into host memory and this DMA could be a read
/modify/write if address is not aligned, instead of a pure write),
 while I tried to align skb to optimize the pktgen tx 
(NIC has to do a DMA to  read packet from host), and align the skb
had no effect.

Maybe we should separate the rx/tx, and try your idea only
for skb allocated for rx.

Also/Or we might try 
__builtin_prefetch (addr, 0, 0);
to instruct cpu to commit to memory cache lines that are
going to be modified by NIC.