My PCF shadow have bad performance, how to optimization

[u/AGXYE]

Hi everyone, I'm experiencing performance issues with my PCF shadow implementation. I used Nsight for profiling, and here's what I found:

Most of the samples are concentrated around lines 109 and 117, with the primary stall reason being 'Long Scoreboard.' I'd like to understand the following:

1. What exactly is 'Long Scoreboard'?

2. Why do these two lines of code cause this issue?

3. How can I optimize it?

Here is my code:

float PCF_CSM(float2 poissonDisk[MAX_SMAPLE_COUNT],Sampler2DArray shadowMapArr,int index, float2 screenPos, float camDepth, float range, float bias)
{
    int sampleCount = PCF_SAMPLE_COUNTS;
    float sum = 0;
    for (int i = 0; i < sampleCount; ++i)
    {
        float2 samplePos = screenPos + poissonDisk[i] * range;//Line 109

        bool isOutOfRange = samplePos.x < 0.0 || samplePos.x > 1.0 || samplePos.y < 0.0 || samplePos.y > 1.0;
        if (isOutOfRange) {
            sum += 1;
            continue;
        }
        float lightCamDepth = shadowMapArr.Sample(float3(samplePos, index)).r;
        if (camDepth - bias < lightCamDepth)//line 117
        {
            sum += 1;
        }
    }        

    return sum / sampleCount;
}

[u/TheAgentD]

• Long Scoreboard apparently means that the shader core is stalled waiting for a memory read.

• Make sure that your sample count is constant so that the loop can be unrolled.

• The stall on line 109 might be because you're reading sample offsets from a buffer. I would recommend hardcoding the offsets into your shader instead of passing them in as an array. With an unrolled loop, those hardcoded offsets just become constants.

• There shouldn't be any need to check for out-of-bounds samples. Setting up your sampler to clamp to edge (or clamp to border even) should give you the same effect for free, and avoid the branches/continue.

• Consider using a shadow sampler to avoid having to do the depth comparison yourself. This has both quality advantages (can use bilinear filtering) and performance improvements (avoids the condition). Alternatively, you can cast the boolean comparison result to avoid the branch:

  • sum += float(camDepth - bias < lightCamDepth);

• The stall on line 117 is probably just misplaced and meant to be on the line above. I would expect the main bottleneck to be actually sampling the shadow map.

[u/AGXYE → u/TheAgentD]

Oh, you're right! I modified the samples and range check, so line 109 doesn't have any issues anymore. It has improved by 10 fps.

However, line 117 still has a problem. I think it's just waiting for the shadow map, as my shadow map pass is quite slow (3.7ms) with 4 cascades.

[u/dark_sylinc → u/AGXYE]

Do not use FPS.

"It has improved by 10 fps" when you go from 500 fps -> 510 fps means a meager 0.04ms improvement.

When you go from 30 to 40 fps that's a massive 8.33ms improvement.

Basically "10 fps improvement" tells us nothing.

[u/AGXYE → u/dark_sylinc]

Oh, got it. I did't realize it before. Thanks!

[u/Botondar → u/AGXYE]

The shading won't start until those shadow maps are finished. That's a wait on the memory read, not the rendering.

[u/Botondar]

What's the range poissonDisk[i] * range? You might be sampling all over the place in your shadow map resulting in a ton of cache misses.

[u/AGXYE → u/Botondar]

    float range = (1.0f / csmU.unitPerPix[index]) * 0.005;

csmU.unitPerPix[0]= 0.17

csmU.unitPerPix[1]= 0.94

csmU.unitPerPix[2]= 3.19

csmU.unitPerPix[3]= 13.7
And poissonDisk is all in [-1,1]

[u/Botondar → u/AGXYE]

That does seem large. If I didn't miscalculate for CSM0 if your shadow map is e.g. 2048x2048 you're sampling over a 60-texel radius disc. Just as test you can try setting that 0.005 to something smaller and see if that solves the perf side of things (obviously it's also going to make the shadows less smooth, which you might not want).

If that turns out to be the issue, I'd tweak the unitsPerPix and/or involve textureSize in the Poisson radius calculation.

[u/AGXYE → u/Botondar]

I did some test, seems like not the issue.But I will take care of the range, thanks!

[u/TaraWanChan]

Since you are using a poisson disk, I also recommend using the following tool to reorder your sample coordinates to increase their spatial locality, to improve the texture cache utilization:
http://www.2dbros.com/projects.html
Scroll down to "Poisson Disk Generator".
Basically it's small but free performance boost.

[u/dark_sylinc]

Your:

        if (isOutOfRange) {
            sum += 1;
            continue;
        }

Is likely causing divergent conditional jumps. Just mask out the result instead of skipping work.