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

[Alan Swanson <swanson@ukfsn.org>]

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.

  http://thread.gmane.org/gmane.comp.video.dri.devel/17274/focus=17305

-- 
Alan.