isn't a particle simulation supposed to be single threaded? why does it look multi-threaded?

[Coz_y_not]

This is a particle simulation using popadvectbyvolume (cached volume, no sim). I was lead to believe that particle simulations are single threaded hence the CPU core clock speed matters more than number of cores.

this however looks quite multi threaded and the parts where some threads go their own way seem to be file handling I think, or am I reading it wrong?

 

 

[Darjeeling]

I think the idea that POP simulations are entirely single-threaded is a bit of an oversimplification.

While it’s true that some parts of DOP/POP (frame stepping, dependency-heavy logic) don’t parallelize well,
nodes like popadvectbyvolume, where each particle independently samples a volume, are quite amenable to multithreading and are implemented that way internally.

Since this setup is using cached volumes, it’s also very likely that what you’re seeing includes:

• volume file loading

• decompression (bgeo.sc or VDB)

• VDB block expansion and conversion into Houdini’s internal structures

All of those can be multi-threaded and will show up as parallel CPU activity.

The parts where a few threads seem to “go their own way” don’t necessarily look like pure file I/O to me;
they could just as well be decompression, memory allocation, or volume data preparation happening alongside the main particle work.

To really pin down what’s dominant, I’d recommend checking Houdini’s Performance Monitor rather than relying only on the OS CPU graphs.
That should make it clear whether the time is spent in popadvectbyvolume itself or in volume loading / preparation.

So in this case, it doesn’t look contradictory to the usual POP behavior — it actually seems like a scenario where core count does help, not just single-core clock speed.