netdev - Re: [RFC] Kernel unaligned access at __skb_flow

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Date:	Fri, 29 Jan 2016 14:28:38 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] Kernel unaligned access at __skb_flow_dissect

On Fri, 2016-01-29 at 14:08 -0800, Alexander Duyck wrote:

> It also means DMA becomes dramatically slower as it introduces a
> partial write access for the start of every frame.  It is why we had
> set NET_IP_ALIGN to 0 on x86 since DMA was becoming more expensive
> when unaligned then reading IP unaligned headers.

Well, I guess that if you have an arch where DMA accesses are slow and
NET_IP_ALIGN = 2, you are out of luck. This is why some platforms are
better than others.


> 
> The gain on recvmsg would probably be minimal.  The only time I have
> seen any significant speed-up for copying is if you can get both ends
> aligned to something like 16B.

On modern intel cpus, this does not matter at all, sure. It took a while
before "rep movsb" finally did the right thing.

memcpy() and friends implementations are much slower on some older
arches (when dealing with unaligned src/dst)

arch/mips/lib/memcpy.S is a gem ;)