GPUComputePassEncoder: dispatchWorkgroups() method

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The dispatchWorkgroups() method of the GPUComputePassEncoder interface dispatches a specific grid of workgroups to perform the work being done by the current GPUComputePipeline (i.e., set via GPUComputePassEncoder.setPipeline()).

Syntax

js
dispatchWorkgroups(workgroupCountX)
dispatchWorkgroups(workgroupCountX, workgroupCountY)
dispatchWorkgroups(workgroupCountX, workgroupCountY, workgroupCountZ)

Parameters

workgroupCountX

The X dimension of the grid of workgroups to dispatch.

workgroupCountY Optional

The Y dimension of the grid of workgroups to dispatch. If omitted, workgroupCountY defaults to 1.

workgroupCountZ Optional

The Z dimension of the grid of workgroups to dispatch. If omitted, workgroupCountZ defaults to 1.

Note: The X, Y, and Z dimension values passed to dispatchWorkgroups() and GPUComputePassEncoder.dispatchWorkgroupsIndirect() are the number of workgroups to dispatch for each dimension, not the number of shader invocations to perform across each dimension. This matches the behavior of modern native GPU APIs, but differs from the behavior of OpenCL. This means that if a GPUShaderModule defines an entry point with @workgroup_size(4, 4), and work is dispatched to it with the call passEncoder.dispatchWorkgroups(8, 8);, the entry point will be invoked 1024 times total — Dispatching a 4 x 4 workgroup 8 times along both the X and Y axes. 4 * 4 * 8 * 8 = 1024.

Return value

None (Undefined).

Validation

The following criteria must be met when calling dispatchWorkgroups(), otherwise a GPUValidationError is generated and the GPUComputePassEncoder becomes invalid:

workgroupCountX, workgroupCountY, and workgroupCountZ are all less than or equal to the GPUDevice's maxComputeWorkgroupsPerDimension limit.

Examples

In our basic compute demo, several commands are recorded via a GPUCommandEncoder. Most of these commands originate from the GPUComputePassEncoder created via beginComputePass().

At the start of the code, we set a global buffer size of 1000. Also, note that the workgroup size in the shader is set to 64.

js
const BUFFER_SIZE = 1000;

// Compute shader
const shader = `
@group(0) @binding(0)
var<storage, read_write> output: array<f32>;

@compute @workgroup_size(64)

...

`;

Later in the code, the dispatchWorkgroups() workgroupCountX parameter is set based on the global buffer size and the shader workgroup count.

js
// …

// Create GPUCommandEncoder to encode commands to issue to the GPU
const commandEncoder = device.createCommandEncoder();

// Initiate compute pass
const passEncoder = commandEncoder.beginComputePass();

// Issue commands
passEncoder.setPipeline(computePipeline);
passEncoder.setBindGroup(0, bindGroup);
passEncoder.dispatchWorkgroups(Math.ceil(BUFFER_SIZE / 64));

// End the render pass
passEncoder.end();

// Copy output buffer to staging buffer
commandEncoder.copyBufferToBuffer(
  output,
  0, // Source offset
  stagingBuffer,
  0, // Destination offset
  BUFFER_SIZE,
);

// End frame by passing array of command buffers to command queue for execution
device.queue.submit([commandEncoder.finish()]);

// …