netdev - Re: [PATCH 05/10] net: move destructor_arg to the front of sk

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Message-ID: <1334132428.5300.2685.camel@edumazet-glaptop>
Date:	Wed, 11 Apr 2012 10:20:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Ian Campbell <ian.campbell@...rix.com>, netdev@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Wei Liu <wei.liu2@...rix.com>, xen-devel@...ts.xen.org
Subject: Re: [PATCH 05/10] net: move destructor_arg to the front of sk_buff.

On Tue, 2012-04-10 at 12:15 -0700, Alexander Duyck wrote:

> 
> Actually now that I think about it my concerns go much further than the
> memset.  I'm convinced that this is going to cause a pretty significant
> performance regression on multiple drivers, especially on non x86_64
> architecture.  What we have right now on most platforms is a
> skb_shared_info structure in which everything up to and including frag 0
> is all in one cache line.  This gives us pretty good performance for igb
> and ixgbe since that is our common case when jumbo frames are not
> enabled is to split the head and place the data in a page.

I dont understand this split thing for MTU=1500 frames.

Even using half a page per fragment, each skb :

needs 2 allocations for sk_buff and skb->head, plus one page alloc /
reference.

skb->truesize = ksize(skb->head) + sizeof(*skb) + PAGE_SIZE/2 = 512 +
256 + 2048 = 2816 bytes


With non split you have :

2 allocations for sk_buff and skb->head.

skb->truesize = ksize(skb->head) + sizeof(*skb) = 2048 + 256 = 2304
bytes

less overhead and less calls to page allocator...

This only can benefit if GRO is on, since aggregation can use fragments
and a single sk_buff, instead of a frag_list



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