PhysX Scratch Buffer in Unity

What is Scratch Buffer in PhysX?

We always encounter bottleneck in scene::simulate function. When there are too many rigid bodies or joints in the scene, a lot of performance is required to complete those simulations.

As far as possible, PhysX will internally allocate temporary buffers
from the scratch memory block, thereby reducing the need to perform
temporary allocations from PxAllocatorCallback.

So… In the functionscene::simulate, scratch buffer is pushed into the scratch allocator. And this means that physX would try to allocate temp buffers inside the scratch buffer.

It needs to be concerned that:

Much of the memory PhysX uses for simulation is held in a pool of blocks, each 16K in size.

Thus we need to make sure that the buffer size should be multiple of 16k. And in addition, this buffer should be 16-byte aligned.

We can use uint8 and malloc to do this.

Scratch Buffer in Unity

Surprisingly, Unity did nothing about scratch buffer(Well… at least for 5.6). It simply passes fixed delta time to physicsScene::simulate function. And this means that it does not use scratch buffer at all.

As a result, Physics.simulate costs too much performance in our rag doll scene. Especially for those low-end device.

And it is time for us to modify the engine code.

Our Modification

We created an int variable in physics manager class. This value means the block number of scratch buffer we need. The default value is 0, which means we still do not use scratch buffer.

Scratch Buffer

We created the scratch buffer in the physics manager constructor function. And destroy this buffer in the destructor function.

Moreover, we add some terminal function to re-allocate scratch buffer during runtime. This makes us easily to compare performance cost between different set of scratch buffer.

And the result is inspiring. Scratch buffer saved us lots of performance for our regular scene.