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

[Michael S. Tsirkin]

On Mon, May 24, 2010 at 09:37:05AM +0300, Avi Kivity wrote:
> On 05/23/2010 07:30 PM, Michael S. Tsirkin wrote:
>>
>>    
>>>> 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).
>>>      
>> So why does it help?
>>    
>
> We actually do access the cacheline several times in a row here (but not  
> in virtio?):
>
>>              case SHARE:
>>                      while (count<  MAX_BOUNCES) {
>>                              /* Spin waiting for other side to change it. */
>>                              while (counter->cacheline1 != count);
>>    
>
> Broadcast a read request.
>
>>                              count++;
>>                              counter->cacheline1 = count;
>>    
>
> Broadcast an invalidate request.
>
>>                              count++;
>>                      }
>>                      break;
>>
>>              case LOCKSHARE:
>>                      while (count<  MAX_BOUNCES) {
>>                              /* Spin waiting for other side to change it. */
>>                              while 
>> (__sync_val_compare_and_swap(&counter->cacheline1, count, count+1)
>>                                     != count);
>>    
>
> Broadcast a 'read for ownership' request.
>
>>                              count += 2;
>>                      }
>>                      break;
>>    
>
> So RMW should certainly by faster using single-instruction RMW  
> operations (or using prefetchw).

Okay, but why is lockunshare faster than unshare?

> -- 
> Do not meddle in the internals of kernels, for they are subtle and quick to 
> panic.