netdev - [PATCH net-next-2.6] net: Increase NET_SKB

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Date:	Wed, 05 May 2010 07:24:09 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>, jamal <hadi@...erus.ca>,
	Tom Herbert <therbert@...gle.com>
Subject: [PATCH net-next-2.6] net: Increase NET_SKB_PAD to 64 bytes

eth_type_trans() & get_rps_cpus() currently need two 64bytes cache lines
in packet to compute rxhash.

Increasing NET_SKB_PAD from 32 to 64 reduces the need to one cache line
only, and makes RPS faster.

NET_IP_ALIGN(2) + ethernet_header(14) + IP_header(20/40) + ports(8)

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 746a652..fe5798b 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1356,9 +1356,12 @@ static inline int skb_network_offset(const struct sk_buff *skb)
  *
  * Various parts of the networking layer expect at least 32 bytes of
  * headroom, you should not reduce this.
+ * With RPS, we raised NET_SKB_PAD to 64 so that get_rps_cpus() fetches span
+ * a 64 bytes aligned block to fit modern (>= 64 bytes) cache line sizes
+ * NET_IP_ALIGN(2) + ethernet_header(14) + IP_header(20/40) + ports(8)
  */
 #ifndef NET_SKB_PAD
-#define NET_SKB_PAD	32
+#define NET_SKB_PAD	64
 #endif
 
 extern int ___pskb_trim(struct sk_buff *skb, unsigned int len);


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