Compute Shader Problem

Gaming and Visualization Technologies APIs DirectX, DXR, DirectCompute
1

I’m implementing a simple N-Body simulation using DX11 & Compute Shader, running on GTX 280, driver version 306.97. Theory behind is based on this article:http://http.developer.nvidia.com/GPUGems3/gpugems3_ch31.html

I also noticed that such simulation is already a part of MS DX SDK (nBodyGravityCS11), where I took some inspiration.

The problem I encountered:

void body_body_interaction(inout float3 ai, float4 bi, float4 bj)
{
	float3 r = bj.xyz - bi.xyz;

    float distSqr = dot(r, r);
    distSqr += g_softeningFactorSq;

	float distInvCube = 1.0f / sqrt(distSqr * distSqr * distSqr);

	//ai += g_FG * bj.w * distInvCube * r; - NOT WORKING
	ai += g_FG *g_fParticleMass * distInvCube * r; //WORKS, g_fParticleMass can be either in cbuffer or global constant, both work
}

Variable bj (xyz - position, w - mass) is at first loaded to shared memory, then GroupMemoryBarrierWithGroupSync() is called to sync group.

[loop]
for(uint block=0; block< num_blocks; ++block)
{
	//Fetch positions to shared cache
	sh_Positions[indexGroup] = oldPar[block * BLOCK_SIZE + indexGroup].pos;
	GroupMemoryBarrierWithGroupSync();

	[unroll]
	for(uint i = 0; i<BLOCK_SIZE; i+=8)
	{
		body_body_interaction(accel, myParticle.pos, sh_Positions[i]);
		body_body_interaction(accel, myParticle.pos, sh_Positions[i+1]);
		body_body_interaction(accel, myParticle.pos, sh_Positions[i+2]);
		body_body_interaction(accel, myParticle.pos, sh_Positions[i+3]);
		body_body_interaction(accel, myParticle.pos, sh_Positions[i+4]);
		body_body_interaction(accel, myParticle.pos, sh_Positions[i+5]);
		body_body_interaction(accel, myParticle.pos, sh_Positions[i+6]);
		body_body_interaction(accel, myParticle.pos, sh_Positions[i+7]);
	}

	GroupMemoryBarrierWithGroupSync();
}

If I use mass stored in bj.w, I end up with NaNs (examined using nSight). Any other way works, using global constant or cbuffer.

Funy thing about this is that if I do the same thing in MS demo, the result is very same - I get no output and buffer contains NaNs. Why am I unable to use 4th vector component from a shared memory in this case?? It is initialized properly on CPU side and the copied to GPU.

Full shader code here:http://pastebin.com/SJhs8ntt

Thank You very much

2

I tried to re-implement above mentioned algorithm using global memory only, and it works, so there is some issue with shared memory I guess. I tried to mess with group size or reduce particle count, result still the same.

Moreover, how is synchronization realized in CS on CPU side? I mean don’t need computed data on CPU, but I re-use them on GPU to position calculated particles using instancing. Which call actually forces the pipeline to wait until CS is finished? Unbindig UAV? Binding SRV (same buffer as UAV)?

3

More update - it must be a driver bug or whatsoever, because the shared memory implementation does work correctly on GTX680, same driver version.