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

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



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