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WebGPU学习(十一):学习两个优化:“reuse render command buffer”和“dynamic uniform buffer offset”

Wonder-YYC 人气:3

大家好,本文介绍了“reuse render command buffer”和“dynamic uniform buffer offset”这两个优化,以及Chrome->webgpu-samplers->animometer示例对它们进行的benchmark性能测试。

上一篇博文:
WebGPU学习(十):介绍“GPU实现粒子效果”

学习优化:reuse render command buffer

提出问题

每一帧经过下面的步骤进行绘制:

  • 创建一个command buffer
  • 开始一个render pass
  • 设置多个render command到command buffer中
  • 结束该render pass

相关代码如下:

return function frame() {
    ...
    const commandEncoder = device.createCommandEncoder();
    ...
    const passEncoder = commandEncoder.beginRenderPass(renderPassDescriptor);
    
    passEncoder.setPipeline(pipeline);
    passEncoder.setVertexBuffer(0, verticesBuffer);
    passEncoder.setBindGroup(0, uniformBindGroup1);
    passEncoder.draw(36, 1, 0, 0);
    
    passEncoder.endPass();
    ...
}

我们可以发现,一般来说,每帧设置的render command不会变化,这造成了重复record的开销。开销具体包括两个方面:

  • js binding的开销,如转换descriptor object(如转换创建render pipeline时传入的参数:GPURenderPipelineDescriptor)和字符串、处理边界、检验数据的合法性等开销
  • 创建render command的开销和设置(encode)render command到command buffer的开销

优化方案

WebGPU提供了GPURenderBundle,只需record一次render command到render bundle,然后每帧执行该bundle,从而实现了command buffer的复用。

WebGPU还支持创建多个bundle,从而可以record不同的render command到对应的render bundle中

案例代码

对案例代码的说明:
1.发起两个drawcall,对应两个bind group。

这里给出原始的案例代码和优化后的案例代码,供读者参考:

  • 原始的案例代码:不使用bundle
    代码如下:
return function frame() {
    ...
    const commandEncoder = device.createCommandEncoder();
    ...
    const passEncoder = commandEncoder.beginRenderPass(renderPassDescriptor);
    passEncoder.setPipeline(pipeline);
    passEncoder.setVertexBuffer(0, verticesBuffer);

    passEncoder.setBindGroup(0, uniformBindGroup1);
    passEncoder.draw(36, 1, 0, 0);

    passEncoder.setBindGroup(0, uniformBindGroup2);
    passEncoder.draw(36, 1, 0, 0);

    passEncoder.endPass();
    ...
}
  • 优化后的案例代码:创建一个bundle
    代码如下:
function recordRenderPass(passEncoder) {
    passEncoder.setPipeline(pipeline);
    passEncoder.setVertexBuffer(0, verticesBuffer);

    passEncoder.setBindGroup(0, uniformBindGroup1);
    passEncoder.draw(36, 1, 0, 0);

    passEncoder.setBindGroup(0, uniformBindGroup2);
    passEncoder.draw(36, 1, 0, 0);
}

const renderBundleEncoder = device.createRenderBundleEncoder({
    colorFormats: [swapChainFormat],
});
recordRenderPass(renderBundleEncoder);
const renderBundle = renderBundleEncoder.finish();


return function frame(timestamp) {
    ...
    const commandEncoder = device.createCommandEncoder();
    ...
    const passEncoder = commandEncoder.beginRenderPass(renderPassDescriptor);

    passEncoder.executeBundles([renderBundle]);

    passEncoder.endPass();
    ...
}
  • 优化后的案例代码:创建两个bundle
    代码如下:
function recordRenderPass1(passEncoder) {
    passEncoder.setPipeline(pipeline);
    passEncoder.setVertexBuffer(0, verticesBuffer);

    passEncoder.setBindGroup(0, uniformBindGroup1);
    passEncoder.draw(36, 1, 0, 0);
}

function recordRenderPass2(passEncoder) {
    passEncoder.setPipeline(pipeline);
    passEncoder.setVertexBuffer(0, verticesBuffer);

    passEncoder.setBindGroup(0, uniformBindGroup2);
    passEncoder.draw(36, 1, 0, 0);
}

const renderBundleEncoder1 = device.createRenderBundleEncoder({
    colorFormats: [swapChainFormat],
});
recordRenderPass1(renderBundleEncoder1);
const renderBundle1 = renderBundleEncoder1.finish();



const renderBundleEncoder2 = device.createRenderBundleEncoder({
    colorFormats: [swapChainFormat],
});
recordRenderPass2(renderBundleEncoder2);
const renderBundle2 = renderBundleEncoder2.finish();


return function frame(timestamp) {
    ...
    const commandEncoder = device.createCommandEncoder();
    ...
    const passEncoder = commandEncoder.beginRenderPass(renderPassDescriptor);

    passEncoder.executeBundles([renderBundle1, renderBundle2]);

    passEncoder.endPass();
    ...
}
}

进一步分析

我们再来看下bundle和render pass相关的定义:

interface GPUDevice : EventTarget {
   ...
   GPURenderBundleEncoder createRenderBundleEncoder(GPURenderBundleEncoderDescriptor descriptor);
   ...
}

dictionary GPURenderBundleEncoderDescriptor : GPUObjectDescriptorBase {
    required sequence<GPUTextureFormat> colorFormats;
    GPUTextureFormat depthStencilFormat;
    //与MSAA有关,这里不分析
    unsigned long sampleCount = 1;
};

...

interface GPUCommandEncoder {
    ...
    GPURenderPassEncoder beginRenderPass(GPURenderPassDescriptor descriptor);
    ...
}

...

dictionary GPURenderPassDescriptor : GPUObjectDescriptorBase {
    required sequence<GPURenderPassColorAttachmentDescriptor> colorAttachments;
    GPURenderPassDepthStencilAttachmentDescriptor depthStencilAttachment;
};

创建bundle时,需要指定与所属render pass相同的color attachments和depthAndStencil attachment的format。

参考资料

Encoder results reuse
Add GPURenderBundle
How do people reuse command buffers?(要FQ)

学习优化:dynamic uniform buffer offset

提出问题

在大多数应用中,每个drawcall需要不同的uniform变量,对应不同的uniform buffer。而uniform buffer被设置在bind group中,这意味着需要在每一帧中为每个drawcall创建并设置一个bind group(或者只创建一次所有的bind group作为cache,然后在每一帧中复用它)。

创建bind group比drawcall的开销更大。通过在“Proposal: Dynamic uniform and storage buffer offsets”中进行的性能测试,我们知道现代图形API创建bind group的个数是有限的(而WebGPU是基于现代图形API而实现的,因此它在WebGPU中也是有限的):

This means, in a single frame, the Metal devices can create 285 bind groups, the D3D12 devices can create 7270 bind groups, and the Vulkan devices can create 18561 bind groups.

优化方案

WebGPU支持“dynamic uniform buffer offset”,也就是说:
可以只创建一个bind group,但是它只能设置一个或多个uniform buffer(不能设置storage buffer等数据);
每个drawcall用对应的offset来设置同一个bind group。

这样就去掉了创建多个bind group的开销。

根据Proposal: Dynamic uniform and storage buffer offsets:

I believe we said:
We need at least one of the two for the MVP
Having both causes more complication because they will fight for root table space so we might have to introduce a combined limit for pushConstantSize + N * DynamicBufferCount.

WebGPU的MVP版本应该不会支持dynamic storage buffer offset,也就是说bind group不能设置storage buffer。

案例代码

对案例代码的说明:
1.bind group只设置一个uniform buffer,它包含的uniform变量为:

float scale;
float offsetX;
float offsetY;
float scalar;
float scalarOffset;

2.一共有100个gameObject,对应100个draw call和uniform变量的100份数据(设置在uniformBufferData中)
3.不同drawcall对应的bind group->uniform buffer的offset需要为256的倍数

这里给出原始的案例代码和优化后的案例代码,供读者参考:

  • 原始的案例代码:创建bind group cache
    代码如下:
const bindGroupLayout = device.createBindGroupLayout({
    bindings: [
        { binding: 0, visibility: GPUShaderStage.VERTEX, type: "uniform-buffer" },
    ],
});


const pipelineLayout = device.createPipelineLayout({ bindGroupLayouts: [bindGroupLayout] });


const pipeline = device.createRenderPipeline({
    layout: pipelineLayout,
    ...
});



const gameObjects = 100;
const uniformBytes = 5 * Float32Array.BYTES_PER_ELEMENT;
const alignedUniformBytes = Math.ceil(uniformBytes / 256) * 256;
const alignedUniformFloats = alignedUniformBytes / Float32Array.BYTES_PER_ELEMENT;

const uniformBuffer = device.createBuffer({
    size: gameObjects * alignedUniformBytes + Float32Array.BYTES_PER_ELEMENT,
    usage: GPUBufferUsage.COPY_DST | GPUBufferUsage.UNIFORM
});


const uniformBufferData = new Float32Array(gameObjects * alignedUniformFloats);

//bind group的cache数组
const bindGroups = new Array(gameObjects);

function setUniformBufferData(i) {
    uniformBufferData[alignedUniformFloats * i + 0] = Math.random() * 0.2 + 0.2;        // scale
    uniformBufferData[alignedUniformFloats * i + 1] = 0.9 * 2 * (Math.random() - 0.5);  // offsetX
    uniformBufferData[alignedUniformFloats * i + 2] = 0.9 * 2 * (Math.random() - 0.5);  // offsetY
    uniformBufferData[alignedUniformFloats * i + 3] = Math.random() * 1.5 + 0.5;       // scalar
    uniformBufferData[alignedUniformFloats * i + 4] = Math.random() * 10;               // scalarOffset
}

for (let i = 0; i < gameObjects; ++i) {
    setUniformBufferData(i);

    bindGroups[i] = device.createBindGroup({
        layout: bindGroupLayout,
        bindings: [{
            binding: 0,
            resource: {
                buffer: uniformBuffer,
                offset: i * alignedUniformBytes,
                size: 5 * Float32Array.BYTES_PER_ELEMENT,
            }
        }]
    });
}

uniformBuffer.setSubData(0, uniformBufferData);


return function frame() {
    ...
    const commandEncoder = device.createCommandEncoder();
    ...
    const passEncoder = commandEncoder.beginRenderPass(renderPassDescriptor);
    passEncoder.setPipeline(pipeline);
    passEncoder.setVertexBuffer(0, verticesBuffer);

    for (let i = 0; i < gameObjects; ++i) {
        passEncoder.setBindGroup(0, bindGroups[i]);
        passEncoder.draw(3, 1, 0, 0);
    }

    passEncoder.endPass();
    ...
}
  • 优化后的案例代码:使用offset
    代码如下:
//设置hasDynamicOffset为true
const dynamicBindGroupLayout = device.createBindGroupLayout({
    bindings: [
        { binding: 0, visibility: GPUShaderStage.VERTEX, type: "uniform-buffer", hasDynamicOffset: true },
    ],
});

const dynamicBindGroup = device.createBindGroup({
    layout: dynamicBindGroupLayout,
    bindings: [{
        binding: 0,
        resource: {
            buffer: uniformBuffer,
            offset: 0,
            size: 5 * Float32Array.BYTES_PER_ELEMENT,
        },
    }],
});


const dynamicPipelineLayout = device.createPipelineLayout({ bindGroupLayouts: [dynamicBindGroupLayout] });

const dynamicPipeline = device.createRenderPipeline({
    layout: dynamicPipelineLayout,
    ...
});

//定义gameObjects等代码与原始的案例代码相同,故省略
...

for (let i = 0; i < gameObjects; ++i) {
    //setUniformBufferData函数与原始的案例代码相同
    setUniformBufferData(i);
}

const dynamicBindGroup = device.createBindGroup({
    layout: dynamicBindGroupLayout,
    bindings: [{
        binding: 0,
        resource: {
            buffer: uniformBuffer,
            offset: 0,
            size: 5 * Float32Array.BYTES_PER_ELEMENT,
        },
    }],
});

uniformBuffer.setSubData(0, uniformBufferData);

const dynamicOffsets = [0];

return function frame() {
    ...
    const commandEncoder = device.createCommandEncoder();
    ...
    const passEncoder = commandEncoder.beginRenderPass(renderPassDescriptor);
    passEncoder.setPipeline(pipeline);
    passEncoder.setVertexBuffer(0, verticesBuffer);


    for (let i = 0; i < gameObjects; ++i) {
        //之所以要预先创建dynamicOffsets数组,然后在这里设置它的元素,而不直接用“passEncoder.setBindGroup(0, dynamicBindGroup, [i * alignedUniformBytes]);”,是因为这样会增加“创建数组”的开销
        dynamicOffsets[0] = i * alignedUniformBytes;
        passEncoder.setBindGroup(0, dynamicBindGroup, dynamicOffsets);
        passEncoder.draw(3, 1, 0, 0);
    }

    passEncoder.endPass();
    ...
}

参考资料

Proposal: Dynamic uniform and storage buffer offsets

性能测试

animometer示例对这两个优化进行了benchmark测试。

(需要说明的是,该示例的“size: 6 * Float32Array.BYTES_PER_ELEMENT”应该被改为“size: 5 * Float32Array.BYTES_PER_ELEMENT”)

该示例的运行截图如下所示:

在右侧的红圈内选中按钮可启用对应的优化;
右上角的紫圈可设置绘制的三角形个数;
在左上角的蓝圈内,第一行显示每一帧在CPU端所用时间,主要包括render pass的js binding所用的时间;第二行显示每一帧总时间,它等于CPU端+GPU端的所用时间。

测试数据

在我的电脑(Mac Pro 2014,MacOS Catalina10.15.1,Chrome Canary 80.0.3977.4)上绘制4万个三角形的测试结果:

  • 只使用bundle与没用任何优化相比

大幅降低了js binding所用时间,由14ms变为0.2ms;
每一帧总时间只降低了20%。

  • 同时使用bundle与offset与只使用bundle相比

js binding所用时间和每一帧总时间几乎没有变化

  • 只使用offset与没用任何优化相比

js binding所用时间大幅增加了60%;
每一帧总时间只稍微增加了10%。

结论

使用offset优化,虽然增加了CPU端开销,但也降低了GPU端开销,从而使每一帧总时间增加得很少。而且它使代码更为简洁(只创建一个bind group),可能也减少了内存占用(我没有进行测试,仅为推测),所以推荐使用。

使用bundle优化,虽然大幅降低了CPU端开销,但也增加了GPU端开销。不过考虑到每一帧总时间还是降低了20%,而且有被浏览器进一步优化的空间(参考Encoder results reuse),所以推荐使用。

参考资料

animometer示例

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