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


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