Subject: Re: tbench regression in 2.6.25-rc1

From: Eric Dumazet <[email-masked]>
Date: Tue, 19 Feb 2008 08:35:36 +0100

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" <[email-masked]> wrote:
>>
>>> 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;
>>>
>>>
>> 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.