Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself

[Michael S. Tsirkin]

On Sun, May 23, 2010 at 07:03:10PM +0300, Avi Kivity wrote:
> On 05/23/2010 06:51 PM, Michael S. Tsirkin wrote:
>>>
>>>> So locked version seems to be faster than unlocked,
>>>> and share/unshare not to matter?
>>>>
>>>>        
>>> May be due to the processor using the LOCK operation as a hint to
>>> reserve the cacheline for a bit.
>>>      
>> Maybe we should use atomics on index then?
>>    
>
> This should only be helpful if you access the cacheline several times in  
> a row.  That's not the case in virtio (or here).
>
> I think the problem is that LOCKSHARE and SHARE are not symmetric, so  
> they can't be directly compared.
>
>> OK, after adding mb in code patch will be sent separately,
>> the test works for my workstation. locked is still fastest,
>> unshared sometimes shows wins and sometimes loses over shared.
>>
>> address@hidden ~]# ./cachebounce share 0 1
>> CPU 0: share cacheline: 6638521 usec
>> CPU 1: share cacheline: 6638478 usec
>>    
>
> 66 ns? nice.
>
>> address@hidden ~]# ./cachebounce share 0 2
>> CPU 0: share cacheline: 14529198 usec
>> CPU 2: share cacheline: 14529156 usec
>>    
>
> 140 ns, not too bad.  I hope I'm not misinterpreting the results.
>
> -- 
> error compiling committee.c: too many arguments to function


Here's another box: here the fastest option
is shared, slowest unshared, lock is in the middle.



address@hidden testring]# sh run 0 2
CPU 2: share cacheline: 3304728 usec
CPU 0: share cacheline: 3304784 usec
CPU 0: unshare cacheline: 6283248 usec
CPU 2: unshare cacheline: 6283224 usec
CPU 2: lockshare cacheline: 4018567 usec
CPU 0: lockshare cacheline: 4018609 usec


CPU 2: lockunshare cacheline: 4041791 usec
CPU 0: lockunshare cacheline: 4041832 usec
address@hidden testring]# 
address@hidden testring]# 
address@hidden testring]# 
address@hidden testring]# sh run 0 1
CPU 1: share cacheline: 8306326 usec
CPU 0: share cacheline: 8306324 usec
CPU 0: unshare cacheline: 19571697 usec
CPU 1: unshare cacheline: 19571578 usec
CPU 0: lockshare cacheline: 11281566 usec
CPU 1: lockshare cacheline: 11281424 usec
CPU 0: lockunshare cacheline: 11276093 usec
CPU 1: lockunshare cacheline: 11275957 usec


address@hidden testring]# sh run 0 3
CPU 0: share cacheline: 8288335 usec
CPU 3: share cacheline: 8288334 usec
CPU 0: unshare cacheline: 19107202 usec
CPU 3: unshare cacheline: 19107139 usec
CPU 0: lockshare cacheline: 11238915 usec
CPU 3: lockshare cacheline: 11238848 usec
CPU 3: lockunshare cacheline: 11132134 usec
CPU 0: lockunshare cacheline: 11132249 usec