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[Manta] Re: Re: Re: Re: information about the acceleration structures?


Chronological Thread 
  • From: Thiago Ize < >
  • To:
  • Cc: Carson Brownlee < >
  • Subject: [Manta] Re: Re: Re: Re: information about the acceleration structures?
  • Date: Sun, 11 Apr 2010 14:21:53 -0600
Packet size should be an issue for certain acceleration structures when you have lots of triangles or incoherent rays, such as in path tracing. If the structure only traces a single ray at a time, then this clearly doesn't matter much, nor does it matter too much if the acceleration structure can gracefully degenerate to a single ray type of performance, which DynBVH can sort of do in some situations (although the state of the art in this has improved since DynBVH was written). Some structures, like the kdtree, do not handle large packets well when each ray wants to traverse different nodes and so in this case smaller packets or even an explicit single ray traversal might work better.

I'm not familiar with GridSpheres, but I would assume that RecursiveGrid would be a much better grid structure than that. RecursiveGrid has the nice property that it scales to really large number of primitives in both performance and while using only linear memory and offering decent traversal performance for a single ray traversal algorithm. Note that the build is not at all optimized so it can be a bit slow.

While you mentioned manta performance compared to gpu rasterizer performance, in the case of gpu ray tracing performance I've come across some interesting results. I've done some light benchmarking of manta running on a nice system (8 nehalem cores) versus a GPU only ray tracer (NVIDIA's Optix) running on a tesla s870 (4 GPUs for total of 2TFlops of performance) all on the same system and found that manta was about 20% faster in all my tests and could scale up to large datasets since it was not restricted to a tiny amount of GPU memory. Of course, you can now get a tesla system that is twice as fast, but the same also goes for the CPUs (plus manta has access to all of the cheap system memory versus the small expensive memory restrictions of GPUs). The take home message is that Manta should be pretty fast if you can run it using SSE and on a machine with many cores. That of course is the important point. If you have a really fancy and expensive graphics card(s) and run manta on a 2 core machine with SSE turned off, then of course manta won't stand a chance.

Thiago

David E DeMarle wrote:
I'll try to post a better (but still informal) breakdown to the manta list
later this week. Here is a sketch of what performance looks like to
me:

The timings are for the MantaBenchmark program in vtkManta that I
wrote, I will export the data to something pure manta can read and
see how that compares this week.

The best accel structure in my benchmark is DynBVH. With that starting
around 50k triangles the framerate starts to plummet downward from
60fps to 30fps at 100k to 6fps at 1million. Playing with max packet
size changes the 1million tri case by about 1fps on either side.

GridSpheres 3 is better between 50k and 100k (it drops barely at all),
but shortly after 100k the build runs out of memory and crashes. The
build time is too long to use in practice for sci-vis, where change is
the norm and pure rendering (just camera motion) comes in bursts.

Manta pretty handily outperforms Mesa GL (it is roughly even for  < 50k
and against a single manta thread), but it never outperforms GL on my
NVidia card. It does start to be competative above 1million tris though.

David E DeMarle
Kitware, Inc.
R&D Engineer
28 Corporate Drive
Clifton Park, NY 12065-8662
Phone: 518-371-3971 x109



On Sun, Apr 11, 2010 at 3:34 AM, Carson Brownlee 
< >
 wrote:
How many triangles are you talking about and would you mind giving a
breakdown of your timings?  I believe Thiagos comment on decreasing packet
size may help if you have a small window size and lot of threads which might
choke the load balancer.  It should not affect single threaded performance
however.
Carson


On Apr 9, 2010, at 5:35 PM, David E DeMarle wrote:

Awesome, thanks.

David E DeMarle
Kitware, Inc.
R&D Engineer
28 Corporate Drive
Clifton Park, NY 12065-8662
Phone: 518-371-3971 x109



On Fri, Apr 9, 2010 at 7:30 PM, Thiago Ize 
< >
 wrote:
BSP and CellSkipper you probably would never want to use since the BSP
has a
super slow build for relatively minor speed gains and CellSkipper is slow
and just interesting from an academic view.

RecursiveGrid is a very good choice when the rays are very incoherent and
single ray traversal is just as good (or better) than packet traversal.
 This happens with path tracing for instance.  The number of levels to
use
is described in my dissertation, but 3 is usually the right amount.  The
build is decent, but could be made much faster if someone spent the time
to
optimize it.

KDTree is ok, but the version in manta is not as optimized as it could
be,
so I'd probably use DynBVH over it.

DynBVH is usually a good acceleration structure to use.  The build can be
made faster by going into cmake and turning
MANTA_USE_DYNBVH_APPROXIMATE   to  ON
however that will result in slower traversal performance.  Faster build
algorithms exist and someone should implement it as well as parallelizing
the build.

The poor scaling with increasing triangles could be a result of the
packet
size being too big. Trying turning it down to see if it improves
performance. The exception to this rule is RecursiveGrid which already
does
a single ray traversal so packet size is not an issue.

If possible, I'd recommend flattening the groups so that there is just a
single group and then building a tree over this since this will give the
best performance. Otherwise, the type of structures to use would depend
on
how the objects are distributed within a group and how the groups are
distributed.

Thiago

David E DeMarle wrote:
Can anyone tell me about, or point me to descriptions of, the ?five?
different acceleration structures (BSP, CellSkipper, RecursiveGrid,
KDTree, RecursiveGrid) in Manta?

vtkManta uses DynBVH, and it appears to be the fastest so far, but it
doesn't seem to scale all that well when the number of triangles
increases. I am wondering if one of the others (with well chosen
settings) would be a better choice.

So far, vtkManta isn't changing the contents of the DynBVH, so the
Dynamic nature of it isn't that important (but quick build times are
important).

We do want to be able to have (groups of) triangles, cylinders and
spheres in the acceleration structure so a non-homogenous acceleration
structure is preferable.

thanks for any pointers,

David E DeMarle
Kitware, Inc.
R&D Engineer
28 Corporate Drive
Clifton Park, NY 12065-8662
Phone: 518-371-3971 x109


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