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SubjectRe: tbench regression in 2.6.25-rc1
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On Fri, 2008-02-15 at 15:22 -0800, David Miller wrote:
> From: Eric Dumazet <dada1@cosmosbay.com>
> 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 <yanmin.zhang@intel.com>

---
--- 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;



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