SubMesh: pre-allocate buffers for _getLinesIndexBuffer

kzhsw · December 27, 2024, 2:04am

Version: 7.42.0

Background

Currently in _getLinesIndexBuffer a linesIndices buffer is initialized as array, which size grows dynamically in the loop after it, which could lead to memory reallocations and more gc pressures. Also, uses array to store numbers would result in at least a 8-bytes double pre spec, and likely 24-bytes “heap number” in some chrome v8.

Code:

github.com

BabylonJS/Babylon.js/blob/fc4533a92114e84ff7eb9c7f464e81bcdc4e9f50/packages/dev/core/src/Meshes/subMesh.ts#L437


      
          public render(enableAlphaMode: boolean): SubMesh {

              this._renderingMesh.render(this, enableAlphaMode, this._mesh._internalAbstractMeshDataInfo._actAsRegularMesh ? this._mesh : undefined);

              return this;

          }

          

          /**

           * @internal

           */

          public _getLinesIndexBuffer(indices: IndicesArray, engine: AbstractEngine): DataBuffer {

              if (!this._linesIndexBuffer) {

                  const linesIndices = [];

          

                  if (indices.length === 0) {

                      // Unindexed mesh

                      for (let index = this.indexStart; index < this.indexStart + this.indexCount; index += 3) {

                          linesIndices.push(index, index + 1, index + 1, index + 2, index + 2, index);

                      }

                  } else {

                      for (let index = this.indexStart; index < this.indexStart + this.indexCount; index += 3) {

                          linesIndices.push(indices[index], indices[index + 1], indices[index + 1], indices[index + 2], indices[index + 2], indices[index]);

                      }

Proposal

Pre-allocate buffers as TypedArray, from the loop below we can know that the size of linesIndices should be exactly 2x the size of this.indexCount (aligned down to 3 elements).
This can also eliminate the overhead of scanning the buffer in engine._normalizeIndexData.

sebavan · December 30, 2024, 10:08pm

Feel free to make a PR for it

kzhsw · December 31, 2024, 1:10am

Hmmm, since iterating the indices buffer is a waste of performance, it it good to determine whether to use u16 or u32 by this.verticesStart + this.verticesCount?

kzhsw · January 2, 2025, 5:18am

github.com/BabylonJS/Babylon.js

SubMesh: pre-allocate buffers for _getLinesIndexBuffer

BabylonJS:master ← kzhsw:patch-1

opened 01:48AM - 02 Jan 25 UTC

kzhsw

+16 -3

## Background Currently in _getLinesIndexBuffer a linesIndices buffer is init…ialized as array, which size grows dynamically in the loop after it, which could lead to memory reallocations and more gc pressures. Also, uses array to store numbers would result in at least a 8-bytes double pre spec, and likely 24-bytes “heap number” in some chrome v8. ## Proposal Pre-allocate buffers as TypedArray, from the loop below we can know that the size of linesIndices should be exactly 2x the size of this.indexCount (aligned down to 3 elements). This can also eliminate the overhead of scanning the buffer in engine._normalizeIndexData. ## References Forum post: <https://forum.babylonjs.com/t/submesh-pre-allocate-buffers-for-getlinesindexbuffer/55585>

sebavan · January 2, 2025, 7:31pm

Thank you !!!

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SubMesh: pre-allocate buffers for _getLinesIndexBuffer

Background

Proposal

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