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SubjectSkb initialization patch

Andrew & David-
Attached is the skbinit patch for 2.5.38 kernel. The results
of SPECWeb99 were posted to lkml using this patch. It showed
1.2% improvement in conforming connections. The results can be
found at:
http://marc.theaimsgroup.com/?l=lse-tech&m=103014598223558&w=2

I posted this patch as an RFC to lkml and the details can be
found here:
http://marc.theaimsgroup.com/?l=linux-kernel&m=102994936831586&w=2

The patch is very simple moving the initialization of skb from
__kfree_skb to alloc_skb. Slab cache preserves the value of
the objects between uses. Looks like we were trying to take
advantage of this feature by initializing before freeing it
and this is good for UNI but for SMP there is no guarantee that
the freed skbs will be given back to the same CPU.

The following are the results running Netperf3:
Pentium III 998 MHz 2-way system
Netperf3 tcp_stream test on an 2-way system using 2.5.38 SMP
kernel. One adapter one connection test with 64k socket buffer
and tcp no-delay ON. NAPI and TSO are disabled.

2.5.38 2.5.40+patch % Improvment
Msg size Throughput Throughput
(bytes) Mbits/sec Mbits/sec
512 568.7 579.8 2.0
1024 629.9 638.5 1.4
2048 673.4 682.1 1.3
4096 720.9 732.3 1.6
8192 779.6 799.8 2.6
16384 809.8 828.9 2.4
32768 757.6 772.1 1.9
65536 743.9 764.8 2.8

2.5.38 kernel baseline profile for 512 msg size - routines
affected by the patch:

c02b09f0 alloc_skb 557
c02b0c40 skb_release_data 879
c02b0cf0 kfree_skbmem 262
c02b0d60 __kfree_skb 1302
Total ticks spent in these routines: 3000


2.5.38 kernel+skbinit patch profile for 512 msg size -
routines affected by the patch:

c02b09f0 alloc_skb 987
c02b0ca0 skb_release_data 671
c02b0d50 kfree_skbmem 262
c02b0dc0 __kfree_skb 1008
Total ticks spent in these routines: 2928

Let me know if you need more data.I realize the performance gain is
not substantial on the two workloads we tested. However the patch is
simple, safe and improves network performance to some extent.

diff -Naur linux-2538s/net/core/skbuff.c
linux-2538skbinit/net/core/skbuff.c
--- linux-2538s/net/core/skbuff.c Wed Sep 25 10:58:25 2002
+++ linux-2538skbinit/net/core/skbuff.c Tue Oct 8 11:38:35 2002
@@ -195,18 +195,38 @@
goto nodata;

/* XXX: does not include slab overhead */
+ skb->next = skb->prev = NULL;
+ skb->list = NULL;
+ skb->sk = NULL;
+ skb->stamp.tv_sec = 0; /* No idea about time */
+ skb->dev = NULL;
+ skb->dst = NULL;
+ memset(skb->cb, 0, sizeof(skb->cb));
+ skb->len = 0;
+ skb->data_len = 0;
+ skb->csum = 0;
+ skb->cloned = 0;
+ skb->pkt_type = PACKET_HOST; /* Default type */
+ skb->ip_summed = 0;
+ skb->priority = 0;
+ atomic_set(&skb->users,1);
+ skb->security = 0; /* By default packets are insecure */
skb->truesize = size + sizeof(struct sk_buff);

/* Load the data pointers. */
skb->head = skb->data = skb->tail = data;
skb->end = data + size;
-
- /* Set up other state */
- skb->len = 0;
- skb->cloned = 0;
- skb->data_len = 0;
-
- atomic_set(&skb->users, 1);
+ skb->destructor = NULL;
+#ifdef CONFIG_NETFILTER
+ skb->nfmark = skb->nfcache = 0;
+ skb->nfct = NULL;
+#ifdef CONFIG_NETFILTER_DEBUG
+ skb->nf_debug = 0;
+#endif
+#endif
+#ifdef CONFIG_NET_SCHED
+ skb->tc_index = 0;
+#endif
atomic_set(&(skb_shinfo(skb)->dataref), 1);
skb_shinfo(skb)->nr_frags = 0;
skb_shinfo(skb)->tso_size = 0;
@@ -220,7 +240,6 @@
goto out;
}

-
/*
* Slab constructor for a skb head.
*/
@@ -328,7 +347,6 @@
#ifdef CONFIG_NETFILTER
nf_conntrack_put(skb->nfct);
#endif
- skb_headerinit(skb, NULL, 0); /* clean state */
kfree_skbmem(skb);
}

@@ -1204,7 +1222,7 @@
sizeof(struct sk_buff),
0,
SLAB_HWCACHE_ALIGN,
- skb_headerinit, NULL);
+ NULL, NULL);
if (!skbuff_head_cache)
panic("cannot create skbuff cache");

Regards,
Mala

Mala Anand
IBM Linux Technology Center
E-mail:mkanand@us.ibm.com
http://www-124.ibm.com/developerworks/opensource/linuxperf
Phone:512-838-8088
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