3 ms·

> unified memory is being massively over hyped. There is a reason GPUs have their own memory bus -- contention. CPU/GPUs fighting over access to memory causes m

> unified memory is being massively over hyped. There is a reason GPUs have their own memory bus -- contention. CPU/GPUs fighting over access to memory causes massive disruption to very parallel computation. Not sure I totally agree with this. Game console have unified memory architecture and it’s a beloved feature. It greatly simplifies things and allows the CPU to far more easily use results computed by the GPU with complicated sync commands or frame delays. Maybe unified architecture is less valuable for non-interactive programs. I’m not sure. This is a fair bit outside of my wheelhouse. Memory access is definitely one of the biggest bottlenecks. So I fully agree with the general concern. And you may even be right that the unified architecture isn’t that interesting. But I’m not so sure it’s the problem you think it is.

angry_octet 6y ago
It does indeed make it easier to code. But you may have noticed that Xbox X has moved to having different speed memories with different memory controllers, even if logically contiguous and uniformly accessible. The CPU pool has a narrower bus and can't be used for graphics objects -- you have to queue for copy to the graphics pool. The driver then schedules a deconflicted copy. The PS5 architecture also has different speeds -- but the slow access is to SSD! It has manual management of streaming resources from SSD to RAM, but also allows direct SSD reads -- but that is a trick, because the SSD controller has a huge RAM cache too. So I'd say we're actually moving away from UMA in general. I think that memory aware scheduling is going to be the next win -- online learning to understand memory access patterns and scheduling compute and cache fill. Fancy cache algorithms used to take too much logic (and slow cache fill logic down), but for SSD->RAM you can do lots of prediction based on program state.
- forrestthewoods 6y ago
  Interesting. Sadly I’ve not gotten to dev with a PS5 or XSX yet. I’d subscribe to your newsletter.

angry_octet 6y ago

It does indeed make it easier to code. But you may have noticed that Xbox X has moved to having different speed memories with different memory controllers, even if logically contiguous and uniformly accessible. The CPU pool has a narrower bus and can't be used for graphics objects -- you have to queue for copy to the graphics pool. The driver then schedules a deconflicted copy. The PS5 architecture also has different speeds -- but the slow access is to SSD! It has manual management of streaming resources from SSD to RAM, but also allows direct SSD reads -- but that is a trick, because the SSD controller has a huge RAM cache too. So I'd say we're actually moving away from UMA in general. I think that memory aware scheduling is going to be the next win -- online learning to understand memory access patterns and scheduling compute and cache fill. Fancy cache algorithms used to take too much logic (and slow cache fill logic down), but for SSD->RAM you can do lots of prediction based on program state.

forrestthewoods 6y ago
Interesting. Sadly I’ve not gotten to dev with a PS5 or XSX yet. I’d subscribe to your newsletter.