[Intel-gfx] [PATCH] [RFC] drm/i915: read-read semaphore optimization

[Ben Widawsky]

On 12/13/2011 09:22 AM, Eric Anholt wrote:
> On Mon, 12 Dec 2011 19:52:08 -0800, Ben Widawsky<ben at bwidawsk.net>  wrote:
>> Since we don't differentiate on the different GPU read domains, it
>> should be safe to allow back to back reads to occur without issuing a
>> wait (or flush in the non-semaphore case).
>>
>> This has the unfortunate side effect that we need to keep track of all
>> the outstanding buffer reads so that we can synchronize on a write, to
>> another ring (since we don't know which read finishes first). In other
>> words, the code is quite simple for two rings, but gets more tricky for
>>> 2 rings.
>>
>> Here is a picture of the solution to the above problem
>>
>> Ring 0            Ring 1             Ring 2
>> batch 0           batch 1            batch 2
>>    read buffer A     read buffer A      wait batch 0
>>                                         wait batch 1
>>                                         write buffer A
>>
>> This code is really untested. I'm hoping for some feedback if this is
>> worth cleaning up, and testing more thoroughly.
>
> You say it's an optimization -- do you have performance numbers?

33% improvement on a hacked version of gem_ring_sync_loop with.

It's not really a valid test as it's not coherent, but this is 
approximately the best case improvement.

Oddly semaphores doesn't make much difference in this test, which was 
surprising.