GLSL compute shader : FPS drop at start, then goes up

Unity Engine
,
1

Hi,

I am writing a GLSL compute shader and encountering a strange behaviour where my GPU utilisation is 100% causing me to drop to ~15FPS, then after ~15 seconds it goes back to normal (low utilization and 60FPS).

Here is the isolated offending snippets (reproduced on a fresh project with just a camera and this, on GLCore graphic backend on W10, rtx3080, Unity 2022.1.0b8.274 and 2021.1.0f1 (tested both))

It’s possible I misunderstood how to use the buffers since I’m new to all of this, which is why I’m asking here before opening a bug report. In this case, please correct me !

GLSL :

#pragma kernel main

GLSLPROGRAM
#version 440
#extension GL_ARB_shading_language_420pack : require

layout(std430, binding = 0) buffer _AlbedosBuffer { vec3 albedos[]; };
layout(std430, binding = 1) buffer _NormalsBuffer { vec3 normals[]; };

layout(local_size_x = 64, local_size_y = 1, local_size_z = 1) in;
void main()
{
    albedos[gl_GlobalInvocationID.x] = vec3(1.0);
    normals[gl_GlobalInvocationID.x] = vec3(1.0, 0.0, 0.0);
}

ENDGLSL

C# :

using UnityEngine;

namespace BugReport
{
    public class Main : MonoBehaviour
    {
        public ComputeShader Shader;

        private ComputeBuffer _AlbedosBuffer;
        private ComputeBuffer _NormalsBuffer;

        // called by Unity when the camera has finished rendering
        private void OnRenderImage(RenderTexture source, RenderTexture destination)
        {
            Shader.SetBuffer(0, "_AlbedosBuffer", _AlbedosBuffer);
            Shader.SetBuffer(0, "_NormalsBuffer", _NormalsBuffer);
            Shader.Dispatch(0, 1024 * 1024 / 64, 1, 1); // dispatch 1Mi threads
        }

        private void OnEnable()
        {
            _AlbedosBuffer = new ComputeBuffer(1024 * 1024, 4 * 32 / 8); // claim 16MiB for 1Mi float3 (which takes as much space as float4)
            _NormalsBuffer = new ComputeBuffer(1024 * 1024, 4 * 32 / 8); // claim 16MiB for 1Mi float3 (which takes as much space as float4)
        }

        private void OnDisable()
        {
            _AlbedosBuffer.Release();
            _AlbedosBuffer = null;  // can help the garbage collector
            _NormalsBuffer.Release();
            _NormalsBuffer = null;  // can help the garbage collector
        }
    }
}

The problem still happens with a single buffer, but it’s less noticeable (50% utilisation and drop to 30 FPS for a few seconds).

Thanks!

2

I get similar problem. I have not found solution yet. I noticed that, GFX memory drops and fps goes up in profiler. Is this related to gpu memory? But even if I release compute buffer right after compute shader finished. This situation still exists.