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

[Ben Widawsky]

On 01/16/2012 01:50 PM, Daniel Vetter wrote:
> On Tue, Dec 13, 2011 at 10:36:15AM -0800, Ben Widawsky wrote:
>> 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.
> 
> Our domain tracking is already complicated in unfunny ways. And (at least
> without a use-case showing gains with hard numbers in either perf or power
> usage) I think this patch is the kind of "this looks cool" stuff that
> added a lot to the current problem.
> 
> So before adding more complexity on top I'd like to remove some of the
> superflous stuff we already have. I.e. all the flushing_list stuff and
> maybe other things ...

Can you be more clear on what exactly you want done before taking a
patch like this? Maybe I can work on it during some down time.

> 
> Cheers, Daniel

~Ben