奇怪的计算着色器延迟
Weird compute shader latency
我正在尝试通过计算着色器进行截头体剔除。为此，我有一对用于实例化顶点属性的缓冲区，以及一对用于间接绘制命令的缓冲区。我的计算着色器检查第一个缓冲区的实例坐标是否在边界体积内，引用第一个绘制缓冲区的计数，subgroupBallot 和 bitCount 以查看子组内的偏移量，然后添加来自其他子组的结果和全局偏移量，最后将结果存储在第二个缓冲区中。全局偏移量存储在第二个间接绘制缓冲区中。
问题是，在负载下，平截头体可能会延迟几帧 (>1) 到移动的相机，边缘有宽线消失的物体。这对我来说似乎很奇怪，因为剔除和渲染是在同一个命令缓冲区中完成的。
在 renderdoc 中进行捕获、使用屏幕截图 alt+printScreen 或暂停呈现线程时，事情会恢复到应有的状态。
我唯一的猜测是，即使开始绘制新帧，过去帧的计算着色器也会继续执行，尽管由于管道障碍，这不应该发生。
着色器代码：
#version 460
#extension GL_KHR_shader_subgroup_ballot : require
struct drawData{
uint indexCount;
uint instanceCount;
uint firstIndex;
uint vertexOffset;
uint firstInstance;
};
struct instanceData{
float x, y, z;
float a, b, c, d;
};
layout(local_size_x = 128, local_size_y = 1, local_size_z = 1) in;
layout(set = 0, binding = 0) uniform A
{
mat4 cam;
vec4 camPos;
vec4 l;
vec4 t;
vec4 r;
vec4 b;
};
layout(set = 0, binding = 1) buffer B
{
uint count;
drawData data[];
} Draw[2];
layout(set = 0, binding = 2) buffer C
{
instanceData data[];
} Instance[2];
shared uint offsetsM[32];
void main()
{
const uint gID = gl_LocalInvocationID.x;
const uint lID = gl_SubgroupInvocationID;
const uint patchSize = gl_WorkGroupSize.x;
Draw[1].data[0] = Draw[0].data[0];//copy data like index count
Draw[1].count = Draw[0].count;
uint offsetG = 0;//accumulating offset within end buffer
uint loops = Draw[0].data[0].instanceCount/patchSize;//constant loop count
for(uint i = 0; i<loops;++i){
uint posa = i*patchSize+gID;//runs better this way for some reason
vec3 pos = camPos.xyz-vec3(Instance[0].data[posa].x, Instance[0].data[posa].y, Instance[0].data[posa].z);//position relative to camera
mat4x3 lrtb = mat4x3(l.xyz, r.xyz, t.xyz, b.xyz);
vec4 dist = pos*lrtb+Model.data[0].rad;//dot products and radius tolerance
bool Pass = posa<Draw[0].data[0].instanceCount&&//is real
(dot(pos, pos)<l.w*l.w) &&//not too far
all(greaterThan(dist, vec4(0))); //within view frustum
subgroupBarrier();//no idea what is the best, put what works
uvec4 actives = subgroupBallot(Pass);//count passed instances
if(subgroupElect())
offsetsM[gl_SubgroupID] = bitCount(actives).x+bitCount(actives).y;
barrier();
uint offsetL = bitCount(actives&gl_SubgroupLtMask).x+bitCount(actives&gl_SubgroupLtMask).y;//offset withing subgroup
uint ii = 0;
if(Pass){
for(; ii<gl_SubgroupID; ++ii)
offsetG+= offsetsM[ii];//offsets before subgroup
Instance[1].data[offsetG+offsetL] = Instance[0].data[posa];
for(; ii<gl_NumSubgroups; ++ii)
offsetG+= offsetsM[ii];}//offsets after subgroup
else for(; ii<gl_NumSubgroups; ++ii)
offsetG+= offsetsM[ii];//same but no data copying
}
if(gID == 0)
Draw[1].data[0].instanceCount = offsetG;
}
对于计算后的渲染通道，我有依赖项：
{//1
deps[1].srcSubpass = VK_SUBPASS_EXTERNAL;
deps[1].dstSubpass = 0;
deps[1].srcStageMask = VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT;
deps[1].dstStageMask = VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT;
deps[1].srcAccessMask = VK_ACCESS_SHADER_WRITE_BIT;
deps[1].dstAccessMask = VK_ACCESS_INDIRECT_COMMAND_READ_BIT;
deps[1].dependencyFlags = 0;
}
{//2
deps[2].srcSubpass = VK_SUBPASS_EXTERNAL;
deps[2].dstSubpass = 0;
deps[2].srcStageMask = VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT;
deps[2].dstStageMask = VK_PIPELINE_STAGE_VERTEX_INPUT_BIT;
deps[2].srcAccessMask = VK_ACCESS_SHADER_WRITE_BIT;
deps[2].dstAccessMask = VK_ACCESS_VERTEX_ATTRIBUTE_READ_BIT;
deps[2].dependencyFlags = 0;
}
命令缓冲区（按原样完全重用，交换链中的每个图像一个）：
vkBeginCommandBuffer(cmd, &begInfo);
vkCmdBindDescriptorSets(cmd, VK_PIPELINE_BIND_POINT_COMPUTE, layoutsPipe[1],
0, 1, &descs[1], 0, 0);
vkCmdBindPipeline(cmd, VK_PIPELINE_BIND_POINT_COMPUTE, pipes[1]);
vkCmdDispatch(cmd, 1, 1, 1);
VkBufferMemoryBarrier bufMemBar[2];
{//mem bars
{//0 indirect
bufMemBar[0].srcAccessMask = VK_ACCESS_SHADER_WRITE_BIT;
bufMemBar[0].dstAccessMask = VK_ACCESS_INDIRECT_COMMAND_READ_BIT;
bufMemBar[0].buffer = bufferIndirect;
bufMemBar[0].offset = 0;
bufMemBar[0].size = -1;
}
{//1 vertex instance
bufMemBar[1].srcAccessMask = VK_ACCESS_SHADER_WRITE_BIT;
bufMemBar[1].dstAccessMask = VK_ACCESS_VERTEX_ATTRIBUTE_READ_BIT;
bufMemBar[1].buffer = bufferInstance;
bufMemBar[1].offset = 0;
bufMemBar[1].size = -1;
}
}
vkCmdPipelineBarrier(cmd, VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT,
VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT, 0, 0, 0, 1, &bufMemBar[0], 0, 0);
vkCmdPipelineBarrier(cmd, VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT,
VK_PIPELINE_STAGE_VERTEX_INPUT_BIT , 0, 0, 0, 1, &bufMemBar[1], 0, 0);
VkRenderPassBeginInfo passBegInfo;
passBegInfo.renderPass = pass;
passBegInfo.framebuffer = chain.frames[i];
passBegInfo.renderArea = {{0, 0}, chain.dim};
VkClearValue clears[2]{{0},{0}};
passBegInfo.clearValueCount = 2;
passBegInfo.pClearValues = clears;
vkCmdBeginRenderPass(cmd, &passBegInfo, VK_SUBPASS_CONTENTS_INLINE);
vkCmdBindDescriptorSets(cmd, VK_PIPELINE_BIND_POINT_GRAPHICS, layoutsPipe[0], 0, 1, &descs[0], 0, 0);
vkCmdBindPipeline (cmd, VK_PIPELINE_BIND_POINT_GRAPHICS, pipes[0]);
VkBuffer buffersVertex[2]{bufferVertexProto, bufferInstance};
VkDeviceSize offsetsVertex[2]{0, 0};
vkCmdBindVertexBuffers(cmd, 0, 2, buffersVertex, offsetsVertex);
vkCmdBindIndexBuffer (cmd, bufferIndex, 0, VK_INDEX_TYPE_UINT32);
vkCmdDrawIndexedIndirectCount(cmd, bufferIndirect, 0+4,
bufferIndirect, 0,
count.maxDraws, sizeof(VkDrawIndexedIndirectCommand));
vkCmdEndRenderPass(cmd);
vkEndCommandBuffer(cmd);
渲染和呈现与两个信号量同步 - imageAvailable 和 renderFinished。 CPU 上的 Frustum 计算顺序正确。验证层已启用。
问题是我缺少主机同步。事实上，即使在同一个命令缓冲区中，也没有主机同步保证（这是有道理的，因为它使我们能够使用事件）。
我正在尝试通过计算着色器进行截头体剔除。为此，我有一对用于实例化顶点属性的缓冲区，以及一对用于间接绘制命令的缓冲区。我的计算着色器检查第一个缓冲区的实例坐标是否在边界体积内，引用第一个绘制缓冲区的计数，subgroupBallot 和 bitCount 以查看子组内的偏移量，然后添加来自其他子组的结果和全局偏移量，最后将结果存储在第二个缓冲区中。全局偏移量存储在第二个间接绘制缓冲区中。
问题是，在负载下，平截头体可能会延迟几帧 (>1) 到移动的相机，边缘有宽线消失的物体。这对我来说似乎很奇怪，因为剔除和渲染是在同一个命令缓冲区中完成的。
在 renderdoc 中进行捕获、使用屏幕截图 alt+printScreen 或暂停呈现线程时，事情会恢复到应有的状态。
我唯一的猜测是，即使开始绘制新帧，过去帧的计算着色器也会继续执行，尽管由于管道障碍，这不应该发生。
着色器代码：
#version 460
#extension GL_KHR_shader_subgroup_ballot : require
struct drawData{
uint indexCount;
uint instanceCount;
uint firstIndex;
uint vertexOffset;
uint firstInstance;
};
struct instanceData{
float x, y, z;
float a, b, c, d;
};
layout(local_size_x = 128, local_size_y = 1, local_size_z = 1) in;
layout(set = 0, binding = 0) uniform A
{
mat4 cam;
vec4 camPos;
vec4 l;
vec4 t;
vec4 r;
vec4 b;
};
layout(set = 0, binding = 1) buffer B
{
uint count;
drawData data[];
} Draw[2];
layout(set = 0, binding = 2) buffer C
{
instanceData data[];
} Instance[2];
shared uint offsetsM[32];
void main()
{
const uint gID = gl_LocalInvocationID.x;
const uint lID = gl_SubgroupInvocationID;
const uint patchSize = gl_WorkGroupSize.x;
Draw[1].data[0] = Draw[0].data[0];//copy data like index count
Draw[1].count = Draw[0].count;
uint offsetG = 0;//accumulating offset within end buffer
uint loops = Draw[0].data[0].instanceCount/patchSize;//constant loop count
for(uint i = 0; i<loops;++i){
uint posa = i*patchSize+gID;//runs better this way for some reason
vec3 pos = camPos.xyz-vec3(Instance[0].data[posa].x, Instance[0].data[posa].y, Instance[0].data[posa].z);//position relative to camera
mat4x3 lrtb = mat4x3(l.xyz, r.xyz, t.xyz, b.xyz);
vec4 dist = pos*lrtb+Model.data[0].rad;//dot products and radius tolerance
bool Pass = posa<Draw[0].data[0].instanceCount&&//is real
(dot(pos, pos)<l.w*l.w) &&//not too far
all(greaterThan(dist, vec4(0))); //within view frustum
subgroupBarrier();//no idea what is the best, put what works
uvec4 actives = subgroupBallot(Pass);//count passed instances
if(subgroupElect())
offsetsM[gl_SubgroupID] = bitCount(actives).x+bitCount(actives).y;
barrier();
uint offsetL = bitCount(actives&gl_SubgroupLtMask).x+bitCount(actives&gl_SubgroupLtMask).y;//offset withing subgroup
uint ii = 0;
if(Pass){
for(; ii<gl_SubgroupID; ++ii)
offsetG+= offsetsM[ii];//offsets before subgroup
Instance[1].data[offsetG+offsetL] = Instance[0].data[posa];
for(; ii<gl_NumSubgroups; ++ii)
offsetG+= offsetsM[ii];}//offsets after subgroup
else for(; ii<gl_NumSubgroups; ++ii)
offsetG+= offsetsM[ii];//same but no data copying
}
if(gID == 0)
Draw[1].data[0].instanceCount = offsetG;
}
对于计算后的渲染通道，我有依赖项：
{//1
deps[1].srcSubpass = VK_SUBPASS_EXTERNAL;
deps[1].dstSubpass = 0;
deps[1].srcStageMask = VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT;
deps[1].dstStageMask = VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT;
deps[1].srcAccessMask = VK_ACCESS_SHADER_WRITE_BIT;
deps[1].dstAccessMask = VK_ACCESS_INDIRECT_COMMAND_READ_BIT;
deps[1].dependencyFlags = 0;
}
{//2
deps[2].srcSubpass = VK_SUBPASS_EXTERNAL;
deps[2].dstSubpass = 0;
deps[2].srcStageMask = VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT;
deps[2].dstStageMask = VK_PIPELINE_STAGE_VERTEX_INPUT_BIT;
deps[2].srcAccessMask = VK_ACCESS_SHADER_WRITE_BIT;
deps[2].dstAccessMask = VK_ACCESS_VERTEX_ATTRIBUTE_READ_BIT;
deps[2].dependencyFlags = 0;
}
命令缓冲区（按原样完全重用，交换链中的每个图像一个）：
vkBeginCommandBuffer(cmd, &begInfo);
vkCmdBindDescriptorSets(cmd, VK_PIPELINE_BIND_POINT_COMPUTE, layoutsPipe[1],
0, 1, &descs[1], 0, 0);
vkCmdBindPipeline(cmd, VK_PIPELINE_BIND_POINT_COMPUTE, pipes[1]);
vkCmdDispatch(cmd, 1, 1, 1);
VkBufferMemoryBarrier bufMemBar[2];
{//mem bars
{//0 indirect
bufMemBar[0].srcAccessMask = VK_ACCESS_SHADER_WRITE_BIT;
bufMemBar[0].dstAccessMask = VK_ACCESS_INDIRECT_COMMAND_READ_BIT;
bufMemBar[0].buffer = bufferIndirect;
bufMemBar[0].offset = 0;
bufMemBar[0].size = -1;
}
{//1 vertex instance
bufMemBar[1].srcAccessMask = VK_ACCESS_SHADER_WRITE_BIT;
bufMemBar[1].dstAccessMask = VK_ACCESS_VERTEX_ATTRIBUTE_READ_BIT;
bufMemBar[1].buffer = bufferInstance;
bufMemBar[1].offset = 0;
bufMemBar[1].size = -1;
}
}
vkCmdPipelineBarrier(cmd, VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT,
VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT, 0, 0, 0, 1, &bufMemBar[0], 0, 0);
vkCmdPipelineBarrier(cmd, VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT,
VK_PIPELINE_STAGE_VERTEX_INPUT_BIT , 0, 0, 0, 1, &bufMemBar[1], 0, 0);
VkRenderPassBeginInfo passBegInfo;
passBegInfo.renderPass = pass;
passBegInfo.framebuffer = chain.frames[i];
passBegInfo.renderArea = {{0, 0}, chain.dim};
VkClearValue clears[2]{{0},{0}};
passBegInfo.clearValueCount = 2;
passBegInfo.pClearValues = clears;
vkCmdBeginRenderPass(cmd, &passBegInfo, VK_SUBPASS_CONTENTS_INLINE);
vkCmdBindDescriptorSets(cmd, VK_PIPELINE_BIND_POINT_GRAPHICS, layoutsPipe[0], 0, 1, &descs[0], 0, 0);
vkCmdBindPipeline (cmd, VK_PIPELINE_BIND_POINT_GRAPHICS, pipes[0]);
VkBuffer buffersVertex[2]{bufferVertexProto, bufferInstance};
VkDeviceSize offsetsVertex[2]{0, 0};
vkCmdBindVertexBuffers(cmd, 0, 2, buffersVertex, offsetsVertex);
vkCmdBindIndexBuffer (cmd, bufferIndex, 0, VK_INDEX_TYPE_UINT32);
vkCmdDrawIndexedIndirectCount(cmd, bufferIndirect, 0+4,
bufferIndirect, 0,
count.maxDraws, sizeof(VkDrawIndexedIndirectCommand));
vkCmdEndRenderPass(cmd);
vkEndCommandBuffer(cmd);
渲染和呈现与两个信号量同步 - imageAvailable 和 renderFinished。 CPU 上的 Frustum 计算顺序正确。验证层已启用。
问题是我缺少主机同步。事实上，即使在同一个命令缓冲区中，也没有主机同步保证（这是有道理的，因为它使我们能够使用事件）。