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

[Jerome Glisse]

On Wed, Nov 28, 2012 at 6:44 PM, Alan Swanson <swanson at 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 at shipmail.org> wrote:
>> > On 11/28/2012 04:58 PM, j.glisse at gmail.com wrote:
>> >>
>> >> From: Jerome Glisse <jglisse at 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.
>
>   http://thread.gmane.org/gmane.comp.video.dri.devel/17274/focus=17305
>
> --
> Alan.

Yes such heuristic might be a good idea, i am working on a prototype
which mostly require a bit of infrastructure.

Cheers,
Jerome