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

[Thomas Hellstrom <thomas@shipmail.org>]

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?

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.


/Thomas







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