3 ms·

For higher power parts, heat dissipation is the primary reason. Most of the high end CPUs are purely heat limited in terms of performance. A TDP of 90W is abou

For higher power parts, heat dissipation is the primary reason. Most of the high end CPUs are purely heat limited in terms of performance. A TDP of 90W is about the limit for CPUs using cheap cooling techniques, and adding ram would lower that by 10-30W.

ericd 8y ago
If you interleaved the memory with the processing, wouldn’t it decrease the heat density, though, which afaik is the limiting factor? You can get rid of quite a lot more heat than that if it’s not in too small an area to conduct away quickly.
- saltcured 8y ago
  The semiconductor processing for high-end DRAM is not the same as for high-end CPU logic or cache SRAM, so it is not cost-effective to put both on the same chip. This was a recurring obstable to past processing-in-memory R&D projects. To mix the two for thermal reasons would lead to some crazy multi-chip modules as in the "chiplet" approach reported recently. However, I suspect you would need far too many chiplets and too many bonds to really give you this thermal benefit rather than just having individual chiplets which are still hot spots with problematic cooling.
- tmd83 8y ago
  I don't think that's feasible either. If you put memory in between chip element that means you now have to route around that adding to wastage. It also means the distance between the elements would increase slowing things down. At the speed that chips are running the physical distance matters (from what I recall).
- mjevans 8y ago
  Processors also tend to run hot, while RAM is more reliable when cool. Much like some parts in human biology benefit from being outside of the core (enough to make it an evolutionary benefit even when it's quite an obvious defect due to vulnerability), having RAM in it's own cooling sub-domain makes sense, and since it's already out there the blade interconnect card design is space, cost, and cooling effective.
nine_k 8y ago
I wonder if the extra computing power achievable by more extensive cooling is worth it, in computation units per watt (including both system power and cooling power). I keep reading that some data centers switch to liquid cooling, so likely it does. Maybe with advances on this front, we'll see the increase of the CPU efficiency that the fabrication process can no more deliver.

ericd 8y ago

If you interleaved the memory with the processing, wouldn’t it decrease the heat density, though, which afaik is the limiting factor? You can get rid of quite a lot more heat than that if it’s not in too small an area to conduct away quickly.

saltcured 8y ago
The semiconductor processing for high-end DRAM is not the same as for high-end CPU logic or cache SRAM, so it is not cost-effective to put both on the same chip. This was a recurring obstable to past processing-in-memory R&D projects. To mix the two for thermal reasons would lead to some crazy multi-chip modules as in the "chiplet" approach reported recently. However, I suspect you would need far too many chiplets and too many bonds to really give you this thermal benefit rather than just having individual chiplets which are still hot spots with problematic cooling.
tmd83 8y ago
I don't think that's feasible either. If you put memory in between chip element that means you now have to route around that adding to wastage. It also means the distance between the elements would increase slowing things down. At the speed that chips are running the physical distance matters (from what I recall).
mjevans 8y ago
Processors also tend to run hot, while RAM is more reliable when cool. Much like some parts in human biology benefit from being outside of the core (enough to make it an evolutionary benefit even when it's quite an obvious defect due to vulnerability), having RAM in it's own cooling sub-domain makes sense, and since it's already out there the blade interconnect card design is space, cost, and cooling effective.

nine_k 8y ago

I wonder if the extra computing power achievable by more extensive cooling is worth it, in computation units per watt (including both system power and cooling power). I keep reading that some data centers switch to liquid cooling, so likely it does. Maybe with advances on this front, we'll see the increase of the CPU efficiency that the fabrication process can no more deliver.