Subject: Re: tbench regression in 2.6.25-rc1

From: "Zhang, Yanmin" <[email-masked]>
Date: Mon, 18 Feb 2008 16:12:38 +0800

On Fri, 2008-02-15 at 15:22 -0800, David Miller wrote:
> From: Eric Dumazet <[email-masked]>
> Date: Fri, 15 Feb 2008 15:21:48 +0100
> 
> > On linux-2.6.25-rc1 x86_64 :
> > 
> > offsetof(struct dst_entry, lastuse)=0xb0
> > offsetof(struct dst_entry, __refcnt)=0xb8
> > offsetof(struct dst_entry, __use)=0xbc
> > offsetof(struct dst_entry, next)=0xc0
> > 
> > So it should be optimal... I dont know why tbench prefers __refcnt being 
> > on 0xc0, since in this case lastuse will be on a different cache line...
> > 
> > Each incoming IP packet will need to change lastuse, __refcnt and __use, 
> > so keeping them in the same cache line is a win.
> > 
> > I suspect then that even this patch could help tbench, since it avoids 
> > writing lastuse...
> 
> I think your suspicions are right, and even moreso
> it helps to keep __refcnt out of the same cache line
> as input/output/ops which are read-almost-entirely :-
I think you are right. The issue is these three variables sharing the same cache line
with input/output/ops.

> )
> 
> I haven't done an exhaustive analysis, but it seems that
> the write traffic to lastuse and __refcnt are about the
> same.  However if we find that __refcnt gets hit more
> than lastuse in this workload, it explains the regression.
I also think __refcnt is the key. I did a new testing by adding 2 unsigned long
pading before lastuse, so the 3 members are moved to next cache line. The performance is
recovered.

How about below patch? Almost all performance is recovered with the new patch.

Signed-off-by: Zhang Yanmin <[email-masked]>

---

--- linux-2.6.25-rc1/include/net/dst.h	2008-02-21 14:33:43.000000000 +0800
+++ linux-2.6.25-rc1_work/include/net/dst.h	2008-02-21 14:36:22.000000000 +0800
@@ -52,11 +52,10 @@ struct dst_entry
 	unsigned short		header_len;	/* more space at head required */
 	unsigned short		trailer_len;	/* space to reserve at tail */
 
-	u32			metrics[RTAX_MAX];
-	struct dst_entry	*path;
-
-	unsigned long		rate_last;	/* rate limiting for ICMP */
 	unsigned int		rate_tokens;
+	unsigned long		rate_last;	/* rate limiting for ICMP */
+
+	struct dst_entry	*path;
 
 #ifdef CONFIG_NET_CLS_ROUTE
 	__u32			tclassid;
@@ -70,10 +69,12 @@ struct dst_entry
 	int			(*output)(struct sk_buff*);
 
 	struct  dst_ops	        *ops;
-		
-	unsigned long		lastuse;
+
+	u32			metrics[RTAX_MAX];
+
 	atomic_t		__refcnt;	/* client references	*/
 	int			__use;
+	unsigned long		lastuse;
 	union {
 		struct dst_entry *next;
 		struct rtable    *rt_next;