Re: [PATCH] drm/ttm: add minimum residency constraint for bo eviction

[Thomas Hellstrom <thomas@shipmail.org>]

On 11/29/2012 01:52 PM, Marek Olšák wrote:
> On Thu, Nov 29, 2012 at 9:04 AM, Thomas Hellstrom <thomas@shipmail.org> wrote:
>> On 11/29/2012 03:15 AM, Marek Olšák wrote:
>>> On Thu, Nov 29, 2012 at 12:44 AM, Alan Swanson <swanson@ukfsn.org> wrote:
>>>> On Wed, 2012-11-28 at 18:24 -0500, Jerome Glisse wrote:
>>>>> On Wed, Nov 28, 2012 at 6:18 PM, Thomas Hellstrom <thomas@shipmail.org>
>>>>> wrote:
>>>>>> On 11/28/2012 04:58 PM, j.glisse@gmail.com wrote:
>>>>>>> From: Jerome Glisse <jglisse@redhat.com>
>>>>>>>
>>>>>>> This patch add a minimum residency time configurable for each memory
>>>>>>> pool (VRAM, GTT, ...). Intention is to avoid having a lot of memory
>>>>>>> eviction from VRAM up to a point where the GPU pretty much spend all
>>>>>>> it's time moving things in and out.
>>>>>>
>>>>>> This patch seems odd to me.
>>>>>>
>>>>>> It seems the net effect is to refuse evictions from VRAM and make
>>>>>> buffers go
>>>>>> somewhere else, and that makes things faster?
>>>>>>
>>>>>> Why don't they go there in the first place instead of trying to force
>>>>>> them
>>>>>> into VRAM,
>>>>>> when VRAM is full?
>>>>>>
>>>>>> /Thomas
>>>>> It's mostly a side effect of cs and validating with each cs, if boA is
>>>>> in cs1 and not in cs2 and boB is in cs1 but not in cs2 than boA could
>>>>> be evicted by cs2 and boB moved in, if next cs ie cs3 is like cs1 then
>>>>> boA move back again and boB is evicted, then you get cs4 which
>>>>> reference boB but not boA, boA get evicted and boB move in ... So ttm
>>>>> just spend its time doing eviction but he doing so because it's ask by
>>>>> the driver to do so. Note that what is costly there is not the bo move
>>>>> in itself but the page allocation.
>>>>>
>>>>> I propose this patch to put a boundary on bo eviction frequency, i
>>>>> thought it might help other driver, if you set the residency time to 0
>>>>> you get the current behavior, if you don't you enforce a minimum
>>>>> residency time which helps driver like radeon. Of course a proper fix
>>>>> to the bo eviction for radeon has to be in radeon code and is mostly
>>>>> an overhaul of how we validate bo.
>>>>>
>>>>> But i still believe that this patch has value in itself by allowing
>>>>> driver to put a boundary on buffer movement frequency.
>>>>>
>>>>> Cheers,
>>>>> Jerome
>>>> So, a variation on John Carmack's recommendation from 2000 to use MRU,
>>>> not LRU, to avoid texture trashing.
>>>>
>>>>     Mar 07, 2000 - Virtualized video card local memory is The Right Thing.
>>>>     http://floodyberry.com/carmack/johnc_plan_2000.html
>>>>
>>>> In fact, this was last discussed in 2005 with a patch for a 1 second
>>>> stale texture eviction and I (still) wondered why a method it was never
>>>> implemented since it was an clear problem.
>>> BTW we can send end-of-frame markers to the kernel, which could be
>>> used to implement Carmack's algorithm.
>>>
>>> Marek
>>
>> It seems to me like Carmack's algorithm is quite specific to the case where
>> only a single GL client is running?
> In theory, we could send context IDs to the kernel as well and modify
> the conditional to "If the LRU texture was not needed in the previous
> frame of any context".
>
>
>> It also seems like it's designed around the fact that when eviction takes
>> place, all buffer objects will be idle. With a
>> reasonably filled graphics fifo / ring, blindly using MRU will cause the GPU
>> to run synchronized.
> I don't see why you would need to synchronize. If the GPU takes care
> of moving buffers in and out of VRAM and there's only one ring buffer
> ==> no synchronization is required.
The LRU bo has a much higher probability of being idle than the MRU bo, 
and waiting for it to become idle will in
principle synchronize the GPU and unnecessarily drain the ring.

/Thomas


> Marek



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