On 11/29/2012 01:52 PM, Marek Ol??k wrote: > On Thu, Nov 29, 2012 at 9:04 AM, Thomas Hellstrom <thomas at shipmail.org> > wrote: >> On 11/29/2012 03:15 AM, Marek Ol??k wrote: >>> On Thu, Nov 29, 2012 at 12:44 AM, 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. >>> 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
