netdev - RE: [PATCH net-next 2/2] bnx2x: uses build

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6D8AEC2@saturn3.aculab.com>
Date:	Mon, 14 Nov 2011 16:21:27 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric Dumazet" <eric.dumazet@...il.com>,
	"David Miller" <davem@...emloft.net>
Cc:	<netdev@...r.kernel.org>,
	"Ben Hutchings" <bhutchings@...arflare.com>,
	"Tom Herbert" <therbert@...gle.com>,
	"Jamal Hadi Salim" <hadi@...atatu.com>,
	"Stephen Hemminger" <shemminger@...tta.com>,
	"Thomas Graf" <tgraf@...radead.org>,
	"Herbert Xu" <herbert@...dor.apana.org.au>,
	"Jeff Kirsher" <jeffrey.t.kirsher@...el.com>,
	<pstaszewski@...are.pl>, <eilong@...adcom.com>
Subject: RE: [PATCH net-next 2/2] bnx2x: uses build_skb() in receive path

 
> bnx2x uses following formula to compute its rx_buf_sz :
> 
> dev->mtu + 2*L1_CACHE_BYTES + 14 + 8 + 8 + 2
> 
> Then core network adds NET_SKB_PAD and SKB_DATA_ALIGN(sizeof(struct
> skb_shared_info))
> 
> Final allocated size for skb head on x86_64 (L1_CACHE_BYTES = 64,
> MTU=1500) : 2112 bytes : SLUB/SLAB round this to 4096 bytes.
> 
> Since skb truesize is then bigger than SK_MEM_QUANTUM, we have lot of
> false sharing because of mem_reclaim in UDP stack.
> 
> One possible way to half truesize is to reduce the need by 64 bytes
> (2112 -> 2048 bytes)

Would seem to be worth trying to reduce the size for systems
where the L1 caches lines are 128 bytes - I think that includes
some of the large ppc systems.

If the alignment of the malloced memory can't be assumed/forced
maybe one of the sub-structures could be dynamically placed
before the data buffer?

	David


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