[PATCH net-next-2.6] pktgen: adding prefetchw() call

[Junchang Wang <junchangwang@gmail.com>]

We know for sure pktgen is going to write skb->data right after
*_alloc_skb, causing unnecessary cache misses.

Idea is to add a prefetchw() call to prefetch the first cache line
indicated by skb->data. On systems with Adjacent Cache Line Prefetch,
it's probably two cache lines are prefetched.

With this prefetch, pktgen on Intel SR1625 server with two E5530 
quad-core processors and a single ixgbe-based NIC went from 8.63Mpps
to 9.03Mpps, with 4.6% improvement.


Signed-off-by: Junchang Wang <junchangwang@gmail.com>
---
 net/core/pktgen.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/net/core/pktgen.c b/net/core/pktgen.c
index 2953b2a..18fe20d 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -2660,6 +2660,7 @@ static struct sk_buff *fill_packet_ipv4(struct net_device *odev,
 		sprintf(pkt_dev->result, "No memory");
 		return NULL;
 	}
+	prefetchw(skb->data);
 
 	skb_reserve(skb, datalen);
 
@@ -3007,6 +3008,7 @@ static struct sk_buff *fill_packet_ipv6(struct net_device *odev,
 		sprintf(pkt_dev->result, "No memory");
 		return NULL;
 	}
+	prefetchw(skb->data);
 
 	skb_reserve(skb, 16);
--

--Junchang