[PATCH net-next-2.6] net: Increase NET_SKB_PAD to 64 bytes

Eric Dumazet
2010-05-05T05:24:15Z

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@gmail.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);