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    DateTue, 19 Feb 2008 08:35:36 +0100
    FromEric Dumazet <>
    SubjectRe: tbench regression in 2.6.25-rc1
    Zhang, Yanmin a écrit :
    > On Mon, 2008-02-18 at 11:11 +0100, Eric Dumazet wrote:
    >> On Mon, 18 Feb 2008 16:12:38 +0800
    >> "Zhang, Yanmin" <yanmin_zhang@linux.intel.com> wrote:
    >>
    >>> 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;
    >>>
    >>>
    >> Well, after this patch, we grow dst_entry by 8 bytes :
    > With my .config, it doesn't grow. Perhaps because of CONFIG_NET_CLS_ROUTE, I don't
    > enable it. I will move tclassid under ops.
    > 
    >> sizeof(struct dst_entry)=0xd0
    >> offsetof(struct dst_entry, input)=0x68
    >> offsetof(struct dst_entry, output)=0x70
    >> offsetof(struct dst_entry, __refcnt)=0xb4
    >> offsetof(struct dst_entry, lastuse)=0xc0
    >> offsetof(struct dst_entry, __use)=0xb8
    >> sizeof(struct rtable)=0x140
    >>
    >>
    >> So we dirty two cache lines instead of one, unless your cpu have 128 bytes cache lines ?
    >>
    >> I am quite suprised that my patch to not change lastuse if already set to jiffies changes nothing...
    >>
    >> If you have some time, could you also test this (unrelated) patch ?
    >>
    >> We can avoid dirty all the time a cache line of loopback device.
    >>
    >> diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
    >> index f2a6e71..0a4186a 100644
    >> --- a/drivers/net/loopback.c
    >> +++ b/drivers/net/loopback.c
    >> @@ -150,7 +150,10 @@ static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
    >>                 return 0;
    >>         }
    >>  #endif
    >> -       dev->last_rx = jiffies;
    >> +#ifdef CONFIG_SMP
    >> +       if (dev->last_rx != jiffies)
    >> +#endif
    >> +               dev->last_rx = jiffies;
    >> 
    >>         /* it's OK to use per_cpu_ptr() because BHs are off */
    >>         pcpu_lstats = netdev_priv(dev);
    >>
    > Although I didn't test it, I don't think it's ok. The key is __refcnt shares the same
    > cache line with ops/input/output.
    > 
    
    Note it was unrelated to struct dst, but dirtying of one cache line of 
    'loopback netdevice'
    
    I tested it, and tbench result was better with this patch : 890 MB/s instead 
    of 870 MB/s on a bi dual core machine.
    
    
    I was curious of the potential gain on your 16 cores (4x4) machine.
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