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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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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