netdev - Question about __alloc

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Message-ID: <20101203101450.GA9573@Desktop-Junchang>
Date:	Fri, 3 Dec 2010 18:14:52 +0800
From:	Junchang Wang <junchangwang@...il.com>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org
Subject: Question about __alloc_skb() speedup

Hi Eric,

I'm reading your patch (ec7d2f2cf3a1 __alloc_skb() speedup),
in which you prefetch skb and the shinfo part. I'm very
curious why we don't prefetch skb->data. It seems that will
help tx path a lot.

I added the following code

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 104f844..c60a808 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -222,6 +222,8 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,

 		child->fclone = SKB_FCLONE_UNAVAILABLE;
 	}
+	prefetchw(data);
+
 out:
 	return skb;
 nodata:

and the pktgen in my server (A Intel SR1625 server with two E5530 
4-core processors and a single ixgbe-based NIC) goes from 7.6Mpps to
8.4Mpps (64 byte), with 10% performance gain.

For rx path, I did experiments on both ixgbe and igb with pktgen+kute,
and there is no change in system performance.

welcome any suggestions and corrections.

Thanks.

--Junchang
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