Hi everyone. Does anyone have experience with several gpu's ? If I use 2x3090 in SLI. Do they add up memory or only 24gb used ?
And does Houdini even support SLI ? I remember there was no support to use 2 gpu's at the same time before.

SLI is only really useful for setting 3DMark performance records. It's not useful for games, and never was useful for professional applications. SLI as a technology is dead. The latest generation omits the connector for it.

There is no SLI in OpenCL, there never was. NVLink, which some might confuse with SLI, can allow cards to pool the memory, but only a few applications support this. I'm not even sure if it is supported on consumer graphics cards like the 3090.

Some applications can use multiple GPUs to split up compute tasks, which each device acting totally separate without any sharing of memory, such as Karma XPU.

I'm not aware any OpenCL accelerated parts of Houdini that can use multiple GPUs on the same work, however it is possible to assign different devices to multiple Houdini sessions to execute different simulations in parallel.

Thank you for an extensive reply. I saw people on redshift forum are saying that 2x3090 with NVlink are pooling the memory so I thought perhaps it would be the case for OpenCl as well.