The single CCD cache stack was a huge problem with the original x3d dual ccd chips (7900/12coreand 7950/16core) as OS’es were not “ccd aware” about which chiplet has the cache. There is a huge access penalty for cache traversal between chiplets as the data has to travel across the infinity fabric rather than inside the CCD only. When the OS would distribute threads across both ccd’s because it only saw threads, you would never get the full benefit of x3d due to loads being placed on cores without x3d and incurring the traversal penalty. This has largely been fixed in Windows and Linux by now, but it still limits the true potential of the non-x3d ccd.
Dual cache stacks would allow both CCD’s to utilize the full benefits of a massive cache rather than just one.
Alk@sh.itjust.works 1 year ago
The main issue is having the 3d cache at all helps performance, and for gaming you’d want your game processes exclusively on the CCD with the 3d cache. So either you disable the other CCD (like windows’ game mode feature) or use software to force your game processes onto only the 3d CCD, which effectively turns the CPU into the 9800x3d anyway. Having processes for a single game split between both CCDs can cause jittering. From what I’ve heard, having a 3d cache on the 2nd CCD can eliminate that jittering, removing the need to manually keep game processes on the first CCD.